Hemoglobin levels measure the concentration of hemoglobin in the bloodstream. According to McCoy's expert, they can be used as an indicator of blood loss.

"Hypoxia" refers to low oxygen; "anoxia" refers to no oxygen.

Dr. Martin Steiner, an expert witness who testified on behalf of Dr. Gunn and OGA, testified that this seizure event was unrelated to Shannon's initial injury. In 2007, Shannon suffered a massive stroke ; Dr. Steiner testified that this too was unrelated to the events of 2004.

OGA filed a motion to disqualify Dr. Gunn's lead trial counsel, Barbara Hilburn, based on its suit against Dr. Gunn for indemnity, which the trial court granted. See In re Gunn , No. 14-13-00566-CV, 2013 WL 5631241 , at *1 (Tex. App.-Houston [14th Dist.] Oct. 15, 2013) (orig. proceeding) (mem. op.). Dr. Gunn sought and obtained mandamus relief from this disqualification order. Id. at *7.

II. Causation

As their first issue, Dr. Gunn and OGA each argue that the evidence was legally insufficient to support the jury's finding that Dr. Gunn's alleged negligence caused Shannon's brain damage. In a legal sufficiency challenge, we consider whether the evidence at trial would enable a reasonable and fair-minded fact finder to reach the verdict under review. City of Keller v. Wilson , 168 S.W.3d 802 , 827 (Tex. 2005). Evidence is legally insufficient to support a jury finding when (1) the record discloses a complete absence of evidence of a vital fact; (2) the court is barred by rules of law or of evidence from giving weight to the only evidence offered to prove a vital fact; (3) the evidence offered to prove a vital fact is no more than a mere scintilla; or (4) the evidence establishes conclusively the opposite of a vital fact. Bustamante v. Ponte , 529 S.W.3d 447 , 455-56 (Tex. 2017) ; King Ranch, Inc. v. Chapman , 118 S.W.3d 742 , 751 (Tex. 2003). The record contains more than a mere scintilla of evidence when the evidence rises to a level that would enable reasonable and fair-minded people to differ in their conclusions. King Ranch, Inc. , 118 S.W.3d at 751 . Conversely, the record contains less than a scintilla when the evidence offered to prove a vital fact's existence is "so weak as to do no more than create a mere surmise or suspicion." Id. All the record evidence must be considered "in the light most favorable to the party in whose favor the verdict has been rendered," and "every reasonable inference deducible from the evidence is to be indulged in that party's favor." Bustamante , 529 S.W.3d at 456 (quoting Merrell Dow Pharmaceuticals, Inc. v. Havner , 953 S.W.2d 706 , 711 (Tex. 1997) ).

Recovery in a medical-malpractice case requires proof to a reasonable medical probability that the injuries complained of were proximately caused by the negligence of a defendant. Columbia Rio Grande Healthcare, L.P. v. Hawley , 284 S.W.3d 851 , 860 (Tex. 2009). Proximate cause includes two components, cause-in-fact and foreseeability. Id. Proof that negligence was a cause-in-fact of the injury requires proof that (1) the negligence was a substantial factor in causing the injury, and (2) without the act or omission, the harm would not have occurred. Id. Thus, to satisfy a legal sufficiency review in such cases, plaintiffs must "adduce evidence of a 'reasonable medical probability' or 'reasonable probability' that their injuries were caused by the negligence of one or more defendants, meaning simply that it is 'more likely than not' that the ultimate harm or condition resulted from such negligence." Bustamante , 529 S.W.3d at 456 (quoting Jelinek v. Casas , 328 S.W.3d 526 , 532-33 (Tex. 2010) ). In medical-malpractice cases, the general rule is that "expert testimony is necessary to establish causation as to medical conditions outside the common knowledge and experience of jurors." Jelinek , 328 S.W.3d at 533 .

The parties offer competing theories on causation. The court of appeals articulated McCoy's theory, which the jury apparently accepted:

McCoy's causation theory is that Gunn failed to adequately treat Shannon's DIC by failing to order FFP to replace Shannon's clotting factors and slow her bleeding, and by failing to infuse enough units of blood. As Shannon continued to bleed, her body attempted various compensation mechanisms in an effort to maintain enough oxygen flow to avoid cardiovascular collapse and damage to her critical organs. However, Shannon continued to lose blood volume. As her blood volume loss approached the critical danger zone of approximately 40%, Shannon's body could no longer compensate, resulting in her cardiac arrest, lack of oxygen flow to her brain, and her permanent brain damage.

489 S.W.3d at 90 . Dr. Gunn and OGA advanced a different theory-that Shannon's DIC caused small blood clots to form in Shannon's vascular system and some of those small clots lodged in blood vessels in Shannon's brain, resulting in her injuries. We summarize the evidence supporting the parties' arguments.

A. McCoy's Theory

For the standard of care and Dr. Gunn's alleged failure to meet it, McCoy relied upon the testimony of Dr. Molly Brewer, a medical doctor board certified in both obstetrics and gynecology and gynecological oncology. Dr. Brewer underwent specific training in handling DIC patients, and she taught "how to handle DIC from placental abruption and other causes" to other ob/gyns.

Dr. Brewer testified that DIC impacts the body's ability to clot, which puts people at risk of bleeding because their clotting factors are no longer working properly. She stated that Shannon "absolutely" had DIC. Dr. Brewer estimated Shannon's blood volume based on her hemoglobin levels, explaining that "for every one point that the hemoglobin goes down, that's approximately equal to one unit of blood." She explained that doctors cannot always look at a patient and know if they are bleeding; therefore, doctors must rely on lab results, specifically hemoglobin levels, to approximate blood volume and blood loss.

Based on this underlying assumption, Dr. Brewer testified that the decreased hemoglobin levels shown in Shannon's lab results indicated that she was losing blood. Specifically, the lab tests ordered at 7:20 a.m. indicated that Shannon's hemoglobin levels had dropped from 9.5 to 5.5, which "should have put [Dr. Gunn] into almost panic mode." Dr. Brewer testified that by 1:00 p.m., Shannon had lost about 33 to 44% of her blood volume. Lab results also indicated that Shannon's PT had increased, meaning that her blood was not clotting properly. Because DIC interfered with Shannon's body's clotting ability, Dr. Brewer stated that it was critical that she receive FFP and that Dr. Gunn breached the standard of care by failing to order or administer FFP in response to Shannon's lab results.

[Y]ou can replace blood until the cows come home, but if you can't clot and you have an open wound like inside the uterus, they're going to continue to bleed. And so you basically put the blood in and the blood comes out, and without the clotting factors, you cannot control this coagulation disorder.

According to Dr. Brewer, a reasonable and prudent physician managing DIC should be making calculations based on hemoglobin levels to justify blood transfusions (e.g., "her hemoglobin went from x to y and therefore, we assume that she's lost two units, three units, four units, and then it's appropriate to replace it"). She testified that Dr. Gunn breached the standard of care by failing to do these calculations and, on a broader level, by failing to create and document a cohesive plan to manage Shannon's DIC.

Additionally, Dr. Brewer believed Shannon's lab results in the morning and the afternoon with respect to her other vital functions should have alerted Dr. Gunn that Shannon was losing blood due to her body's inability to clot. Dr. Brewer reported that Shannon's increased creatinine levels indicated that she had "acute renal failure," which, read in the context of DIC, tells us "[t]hat [she] probably doesn't have enough blood flow through [the] kidneys." She also pointed to Shannon's decreased oxygen saturation levels and her increased heart rate, describing these phenomena as compensation mechanisms to deal with low blood volume. "[I]f their blood is low, their heart rate goes up to get more blood.... If their oxygen is low, they breathe harder because it brings in more oxygen.... [T]hey actually shunt blood away from their kidneys because they want to spare the heart and ... the brain." Dr. Brewer stated that Shannon's low urine output was another sign of decreased blood flow to the kidneys, which should have been a warning sign. Instead of properly treating Shannon's low blood volume, Dr. Gunn administered Lasix, which is "for fluid overload" and probably made it worse for Shannon. She testified that a reasonable and prudent physician needs to stay equal with or ahead of blood loss in managing a patient like Shannon, and in this case, "Shannon was behind in her blood volume all the way along."

According to Dr. Brewer, Dr. Gunn's failure to replace Shannon's clotting factors by administering FFP contributed to her developing uterine atony. In the afternoon, Shannon became agitated. Dr. Brewer described this as one of the cardinal signs "that she's going into cardiovascular collapse. I mean, something terrible has happened. Patients get agitated when they don't get enough oxygen." When asked about Shannon's oxygen saturation level of 72%, Dr. Brewer responded, "I think it's a crisis." Dr. Brewer testified that after about 1:00 p.m., "the d[i]e was cast with reasonable certainty. I mean, they were in serious trouble. This woman was dying in front of their eyes. I mean, she had blood pouring out." In looking at the whole picture and at the blood loss, Dr. Brewer said it was foreseeable that Shannon would go into ventricular fibrillation. "The problem was [ ] she had lost so much blood at that point that it was a disaster."

Dr. Brewer stated that Shannon's lack of blood led to her ventricular fibrillation in the operating room. When a patient goes into ventricular fibrillation, her heart stops pumping blood and she is unable get oxygen to the brain. Shannon's EEG results indicated, "No focal or epileptic features are noted." Dr. Brewer testified that "the way that a reasonable person would interpret that is that something had happened to all of the brain.... A global injury." At St. Luke's there were seventeen neurologic consults, and each neurologist agreed that Shannon had anoxic encephalopathy, which is global damage to the brain caused by a lack of oxygen.

Dr. Brewer offered the following conclusions: Dr. Gunn was negligent in failing to give more blood products from the beginning; Dr. Gunn was negligent in failing to order more frequent lab tests to determine whether the blood products needed to be adjusted; and Dr. Gunn was negligent in failing to include adjustments to the administration of blood products to deal with the hemoglobin, platelets, and lack of FFP. "You can [pour] the blood in, but the blood just keeps coming out if you can't clot it."

B. Dr. Gunn and OGA's Theory

Dr. Gunn and OGA produced three experts on the issue of liability: Dr. James Aubuchon, a medical doctor, board certified in anatomic and clinical pathology, blood banking, and transfusion medicine; Dr. James Alexander, a medical doctor, board certified in obstetrics and gynecology with a subspecialty of maternal/fetal medicine; and Dr. Martin Steiner, a neurologist.

Contrary to Dr. Brewer's testimony, Dr. Aubuchon testified that with the blood products and fluids she had been given, Shannon had more than enough volume for her heart to function properly. He testified that her lab results (hemoglobin, creatinine, and PT), while outside of the normal ranges, were not "exceedingly high" or cause for panic. He also pointed to nurses' notes that recorded the presence of clots in the post-delivery vaginal bleeding, which "implies that the clotting system [was] able to work."

The three experts collectively offered an alternative theory of liability. They testified that placental abruption is a result of a rupture of the mother's blood vessels in the uterus. This leads to the exposure of a substance known as thromboplastin. Thromboplastin is intended normally to start a clotting process in a small area, but in the case of placental abruption, it gets into the blood system and kicks off the clotting system inside the blood vessels themselves. This process is effectively DIC: an activation of the clotting system, such that small clots, or "thrombi," are being formed throughout the entire body. These small clots move through the vascular system, and if they move into even smaller vessels, they may block those vessels, which is called "microthrombus." Such blockage would prevent blood flow to surrounding tissues, causing those tissues to die from lack of oxygen.

Dr. Steiner testified that Shannon's computerized axial tomography (CAT) scans after the injury were not consistent with a global hypoxic injury, but instead showed results focused in only a small area of the brain. Thus, he opined that "it's these microthrombi or these small little [clots] that we have been talking about" that cause the type of problems Shannon experienced. "It's not the type of thing that you see when there is a global hypoxia or a global lack of oxygen to the brain." He concluded that instead of a global or hypoxic event, Shannon suffered ischemia, or lack of blood flow to tissue, to small areas of her brain from these micro clots, and her neurological damage was "due to the small little microthrombi blocking off the blood vessels and causing teeny strokes." In support of this theory, he noted Shannon's initial improvement, which is typical in a patient who has mini strokes, but not in a patient who has suffered global problems of hypoxia to the brain. Thus, Dr. Steiner testified that Shannon's seizure in 2005, which he opined was unrelated to her initial brain injury, was significant because Shannon never regained the improved function she initially experienced.

C. Competency of Evidence

In analyzing whether there was legally sufficient evidence of causation, we start with the general proposition that "a determination of scientific reliability is appropriate in reviewing the legal sufficiency of evidence." Merck & Co. v. Garza , 347 S.W.3d 256 , 262 (Tex. 2011).

Justice Gonzalez, in writing for the Court, gave rather colorful examples of unreliable scientific evidence in E.I. du Pont de Nemours & Co. v. Robinson , 923 S.W.2d 549 , 558 (Tex. 1995), when he said that even an expert with a degree should not be able to testify that the world is flat, that the moon is made of green cheese, or that the Earth is the center of the solar system. If for some reason such testimony were admitted in a trial without objection, would a reviewing court be obliged to accept it as some evidence? The answer is no.

Havner , 953 S.W.2d at 712 . Examination of an expert's underlying methodology is "a task for the trial court in its role as gatekeeper, and [is] not an analysis that should be undertaken for the first time on appeal."

Coastal Transp. Co. v. Crown Cent. Petroleum Corp. , 136 S.W.3d 227 , 233 (Tex. 2004). However, we have long held that incompetent evidence is legally insufficient to support a judgment, even if admitted without objection. City of Keller , 168 S.W.3d at 812 ; see also Coastal Transp. Co. , 136 S.W.3d at 233 (holding that a legal sufficiency challenge may be brought on appeal even when there was no admissibility objection to the reliability of the expert's opinion; in that case, the challenge is limited to the face of the record, "for example, when the expert testimony is speculative or conclusory on its face"); Hous. Unlimited, Inc. Metal Processing v. Mel Acres Ranch , 443 S.W.3d 820 , 829 (Tex. 2014) (noting that an expert opinion admitted without objection may be probative evidence even if its basis is unreliable, but such opinion offered with no basis or with a basis that provides no support "is merely a conclusory statement and cannot be considered probative evidence, regardless of whether there is no objection"). Thus, evidence showing an expert's opinion to be incompetent cannot be disregarded, even if the result is contrary to the verdict. City of Keller , 168 S.W.3d at 812 . And if an expert's opinion is based on certain assumptions about the facts, we must consider evidence showing those assumptions were unfounded. Id. at 813 ; see also Mel Acres Ranch , 443 S.W.3d at 829 (explaining that reliance on insufficient data and unsupported assumptions, as well as analytical gaps, can render an expert's opinion conclusory and without any evidentiary value). Thus, "if no basis for the opinion is offered, or the basis offered provides no support, the opinion is merely a conclusory statement and cannot be considered probative evidence, regardless of whether there is no objection." Mel Acres Ranch , 443 S.W.3d at 829 .

1. The Hemoglobin Test

Dr. Gunn challenges the underlying basis for Dr. Brewer's opinion-that one point on the hemoglobin test is equivalent to one unit of blood. 5 At trial, Dr. Aubuchon testified that a hemoglobin test measures the concentration of hemoglobin in the blood, but "[i]t doesn't by itself say anything about how much blood the patient has." He testified that hemoglobin levels are impacted by a patient's hydration status: "If you were given a lot of IV fluids, that would cause your hemoglobin level, your concentration of hemoglobins, to go down." Thus, when Shannon's hemoglobin levels dropped to 5.5, he believed it was as a result of at least four liters of IV fluids she received prior to that. Moreover, Dr. Gunn argues that Dr. Brewer's testimony is contradictory because Shannon's lab results at 1:16 p.m. indicated that her hemoglobin levels had in fact increased from 5.5 to 7.5.

An expert's opinion may be considered unreliable if it is based on assumed facts that vary materially from the actual facts, or if it is based on tests or data that do not support the conclusions reached. Whirlpool Corp. v. Camacho , 298 S.W.3d 631 , 637 (Tex. 2009) ; see Mel Acres Ranch , 443 S.W.3d at 833 . In either instance, the opinion is not probative evidence. Camacho , 298 S.W.3d at 637 . However, this does not mean that an expert's factual assumptions must be uncontested or established as a matter of law-if the evidence conflicts, it is normally the province of the jury to determine which evidence to credit. Mel Acres Ranch , 443 S.W.3d at 833 . But if the record contains no evidence supporting an expert's material factual assumptions, or if such assumptions are contrary to conclusively proven facts, opinion testimony founded on those assumptions is not competent evidence. Id. Expert testimony may also be unreliable if "there is simply too great an analytical gap between the data [relied upon] and the opinion proffered." Id. at 835 (quoting Gammill v. Jack Williams Chevrolet, Inc. , 972 S.W.2d 713 , 726 (Tex. 1998) ). We are not required to ignore fatal gaps in an expert's analysis or assertions that are simply incorrect, and such a flaw in an expert's reasoning renders the scientific testimony "unreliable and, legally, no evidence." Id. (quoting Havner , 953 S.W.2d at 714 ).

In her testimony, Dr. Brewer first offered a basis for using hemoglobin as an indicator of blood loss:

Q: Now you hear about these blood tests like a CBC, a complete blood count. Is a hemoglobin included in the CBC?

A: It is. Usually, a hemoglobin, hematocrits. It looks at white blood cells, it look[s] at red cells ... and it looks at platelet count.

Q: If a physician wants to know if his or her patient is bleeding, can they order a CBC and look at hemoglobin ?

A: They can. They can also just order a hemoglobin and a hematocrit. They can do either.

She testified that using lab results, including hemoglobin, to measure blood loss is necessary because doctors "[a]bsolutely" cannot always look at a patient and know if she is bleeding. And Dr. Brewer explained the inconsistency in Shannon's increased hemoglobin level in the afternoon.

Q: So what really was going on when the hemoglobin went from 5.5 at 7:27 in the morning to 7.5 at 1:16 in the afternoon, explain that to us please.

A: That she was continuing to bleed.

Q: And how do you know that?

A: Because from the amount that was put in, that she was infused, it should have been higher than that.

Finally, Dr. Brewer acknowledged that hemoglobin levels provide only an approximation of blood loss, repeatedly emphasizing on both direct and cross examinations that such levels must be viewed within the broader context of Shannon's condition.

Q: ... But if [Shannon's hemoglobin ] was 5.5, that fact alone with no other evidence, you would agree one would not expect a patient to have a cardiac arrest just by having a 5.5 hemoglobin. Correct?

A: I can't agree with what you're saying because you're taking things out of context.

Q: Let's do this as-let's don't use Shannon McCoy.

A: I don't think you can say it about any patient. It has to be in the context of what's happening medically.

The court of appeals correctly noted that the jury was entitled to credit Dr. Brewer's testimony "that it is not 'appropriate' for someone managing a DIC case to consider just one lab result such as hemoglobin and ignore all the other markers, including the presence of quantifiable external bleeding." 489 S.W.3d at 93 (quoting Dr. Brewer's testimony). Thus, while the assumption underlying Dr. Brewer's opinion is not uncontested or established as a matter of law, it is also not unfounded or scientifically unreliable on the face of the record, and the jury was free to credit both the assumption and the opinion resting on it. City of Keller , 168 S.W.3d at 827 (holding that a reviewing court "must credit favorable evidence if reasonable jurors could and disregard contrary evidence unless reasonable jurors could not").

2. The Quantity of Blood Products Transfused

Applying Dr. Brewer's hemoglobin assumption, the court of appeals conducted detailed calculations of Shannon's blood loss based on the evidence in the record, reaching the following conclusions:

Utilizing the initial blood volume figure of 20.1 units cited by Gunn and OGA, the jury reasonably could have concluded that Shannon's blood volume loss reached approximately 36% by 1:00 p.m. in reliance on these figures:

• 20.1 units of blood present in Shannon's body at 11:00 a.m. on September 13 minus 13.2 units of blood lost plus 6 units of blood replaced equals 12.9 units of blood present at 1:00 p.m. on September 14.

• 20.1 units of blood present in Shannon's body at 11:00 a.m. on September 13 minus 12.9 units of blood present at 1:00 p.m. on September 14 equals a net blood loss of 7.2 units during that interval.

• A net blood loss of 7.2 units as of 1:00 p.m. on September 14 divided by 20.1 units of blood present at 11:00 a.m. on September 13 equals 36% of blood volume loss during that interval.

This percentage falls squarely within Brewer's estimated blood volume loss range of 33% to 44%, and approaches the critical danger zone where it is undisputed on this record that cardiac arrest is a reasonable medical probability.

489 S.W.3d at 91-92 . Dr. Gunn and OGA each argue that the court of appeals' assertion that Shannon received only "6 units of blood" is contrary to the record, and that prior to 1:00 p.m., she actually received at least 12 units of blood products. See id. at 92 . This inconsistency reflects the confusion between "blood" and "blood products," implicating a quality-versus-quantity debate that persisted during oral argument before this Court. The court of appeals, relying on Dr. Brewer's hemoglobin assumption, included only the transfusions of packed red blood cells in its reference to "blood," see id. , while Dr. Gunn and OGA's calculation of "12 units of blood products" includes the two units of FFP and the four units of platelets that were administered in addition to the packed red blood cells. Dr. Brewer's theory operates under the apparent assumption that blood volume is primarily increased by transfusions of packed red blood cells, whereas transfusions of FFP and platelets have less effect on volume, but instead are administered to promote the clotting process. 6 Again, here, it is the province of the jury to determine which evidence to credit, Mel Acres Ranch , 443 S.W.3d at 833 , and we review evidence and inferences in the light most favorable to the jury's findings. Bustamante , 529 S.W.3d at 456 .

D. Superiority Requirement

To avoid being conclusory, an expert must, to a reasonable degree of medical probability, explain how and why the negligence caused the injury. Jelinek , 328 S.W.3d at 536 . Importantly, when the evidence demonstrates that "there are other plausible causes of the injury or condition that could be negated, the plaintiff must offer evidence excluding those causes with reasonable certainty." Bustamante , 529 S.W.3d at 456 (quoting Havner , 953 S.W.2d at 720 ) (no emphasis in original). Thus, when the facts support several possible conclusions, only some of which establish that the defendant's negligence caused the plaintiff's injury, the expert must explain to the fact finder why those conclusions are superior based on verifiable medical evidence, not simply the expert's opinion. Jelinek , 328 S.W.3d at 536 . OGA asserts that the record contains no evidence that Dr. Brewer's blood-loss theory was superior to Dr. Aubuchon, Dr. Alexander, and Dr. Steiner's DIC/blood-clot theory. We disagree.

McCoy's counsel addressed Jelinek 's superiority requirement head-on, asking Dr. Brewer directly to explain why her theory was medically superior to the theory advanced by the defense experts. 7 In response, Dr. Brewer talked at length about what she perceived as gaps in Dr. Alexander's and Dr. Steiner's testimony. She also explained that Shannon's EEG was consistent with a global brain injury because it lacked the focal features that would be consistent with mini strokes to a specific area of the brain. This conclusion was consistent with the unanimous opinions of the seventeen neurologists who assessed and treated Shannon in the months following her injury. 8 Dr. Brewer testified that the microthrombi study relied on by Dr. Steiner was invalid as applied to Shannon because it involved a patient population that already had brain damage, and thus was too dissimilar from Shannon to provide a useful comparison. And she testified that Shannon's lack of clotting based on her lab results rendered the defense's microthrombi theory "highly unlikely." Based on the record, Dr. Brewer's testimony more than satisfied Jelinek 's superiority requirement. See id.

Like many medical-malpractice cases, this case involved a battle of the experts. See, e.g. , id. at 533 . In such cases, jurors are the sole judges of the credibility of the witnesses and the weight to give their testimony. City of Keller , 168 S.W.3d at 819 . It is the province of the jury to resolve conflicts in the evidence, and when reasonable jurors could resolve conflicting evidence either way, we presume they did so in accordance with the verdict. Id. at 820 . The jury in this case heard conflicting expert opinions and it reasonably could have believed Dr. Brewer's testimony in light of the evidence. Id. ; see also Mel Acres Ranch , 443 S.W.3d at 833 . We agree with the court of appeals that the evidence is legally sufficient to support a finding of proximate cause.

III. Shannon's Future Medical Expenses

Both Dr. Gunn and OGA assert that the trial court erred in refusing to admit video deposition testimony of Dr. Helen Schilling, which Dr. Gunn and OGA offered to controvert McCoy's evidence of the cost of future medical expenses. 9 We review a trial court's exclusion of an expert witness's testimony for an abuse of discretion. Caffe Ribs, Inc. v. State , 487 S.W.3d 137 , 142 (Tex. 2016). A trial court abuses its discretion by failing to follow guiding rules and principles. Hawley , 284 S.W.3d at 856 . The court of appeals did not address the propriety of the decision to exclude Dr. Schilling's deposition testimony, concluding that even if its exclusion was in error, the error was harmless. 489 S.W.3d at 111 . To reverse a trial court's judgment based on the exclusion of evidence, we must find that the trial court did in fact commit error, and that the error was harmful. Gee v. Liberty Mut. Fire Ins. Co. , 765 S.W.2d 394 , 396 (Tex. 1989).

We first assess whether Dr. Schilling's exclusion was in error and conclude that it was. At trial, McCoy presented evidence of future medical expenses through the testimony of Dr. Alex Willingham. When Dr. Gunn and OGA sought to present video deposition testimony from their own damages expert, Dr. Schilling, McCoy objected on the ground that the video cuts at issue did not contain the predicate to establish Dr. Schilling's qualifications as an expert witness. The trial court agreed and refused to allow the video deposition cuts on their own, but it acknowledged that Dr. Schilling could testify live at trial to establish the basis for her expertise. Dr. Gunn and OGA chose not to call Dr. Schilling as a live witness, but presented an offer of proof instead.

To testify as an expert, a witness must be qualified, and the proposed testimony must be relevant to the issues in the case and based upon a reliable foundation. TEX. R. EVID. 702 ; Robinson , 923 S.W.2d at 556 . Therefore, to be admissible, the specific video cuts at issue needed to contain Dr. Schilling's qualifications. If a court ruling excludes evidence, a party must preserve error by filing an offer of proof informing the court of the substance of the excluded evidence. TEX. R. EVID. 103. Dr. Gunn and OGA's offer of proof shows that Dr. Schilling's deposition provides adequate testimony as to her qualifications, including details about her background in physical medicine and rehabilitation, her experience teaching residents in that field, her experience as a director of rehabilitation at a hospital, her experience preparing life-care plans as part of her daily practice, and her own examination of Shannon. 10 Based on the offer of proof, which references the transcript of the excluded testimony, Dr. Schilling's qualifications are not dissimilar from those of Dr. Willingham, whose testimony was admitted without objection. Both doctors completed a physical medicine and rehabilitation residency, served as medical directors at rehabilitation centers, are on a faculty teaching physical medicine and rehabilitation residents, and prepare life-care plans as part of their practices. Of course, without the specific video cuts in the record, we cannot determine with absolute certainty that the qualifications provided in the offer of proof (Dr. Schilling's deposition) were actually contained in the proffered testimony. To be clear, an offer of proof is not a work-around for the foundational requirement that an expert's qualifications be proven, and it is limited to the substance of the excluded evidence. See TEX. R. EVID. 103. However, based on the record before us, we are satisfied that the defense offered testimony of an expert witness whose qualifications were established. We have no reason to believe that the defense excluded its own expert witness's qualifications from the video cuts offered, 11 nor can we conclude from the record before us that the qualifications discussed in the defense's offer of proof were in fact excluded from the video cuts that it intended to show the jury. 12

Litigants in Texas are afforded a broad right to make strategic decisions when introducing evidence at trial, and they are entitled to present experts in a manner of their choosing, so long as it is consistent with the Texas Rules of Civil Procedure and the Texas Rules of Evidence. E.g. , Jones v. Colley , 820 S.W.2d 863 , 866 (Tex. App.-Texarkana 1991, writ denied) ("A party, as a matter of trial strategy, is entitled to present his evidence in the order he believes constitutes the most effective presentation of his case, provided that it does not convey a distinctly false impression."). Our rules make deposition testimony admissible without regard to the party's availability to appear live. TEX. R. CIV. P. 203.6(b) ("All or part of a deposition may be used for any purpose in the same proceeding in which it was taken."); TEX. R. EVID. 801(e)(3) (indicating that a deponent's statement is not hearsay and "[t]he deponent's unavailability as a witness is not a requirement for admissibility"). Texas Rule of Civil Procedure 203.6 allows any part of a deposition to be presented in evidence, and no rule requires that the deposition be entered in its entirety. TEX. R. CIV. P. 203.6(b). On the contrary, the rule of optional completeness offers the adverse party a remedy if their sole objection to the deposition testimony is its completeness. TEX. R. EVID. 106 ("If a party introduces all or part of a writing or recorded statement, an adverse party may introduce, at that time, any other part ... that in fairness ought to be considered at the same time. 'Writing or recorded statement' includes depositions."); Jones , 820 S.W.2d at 866 . OGA's attorney indicated at the time of the objection that Dr. Schilling's qualifications were contained in the proffered video cuts, and considering the deposition transcript, we see no reason to doubt that representation. Moreover, Dr. Schilling's qualifications were established in Dr. Gunn and OGA's offer of proof, referencing page and line numbers of the same deposition testimony they sought to present by video. Accordingly, we hold that the trial court abused its discretion in excluding Dr. Schilling's testimony.

We next consider whether the exclusion of this evidence was harmful. 13 Texas Rule of Appellate Procedure 61.1 recites our reversible error standard-under that standard, we may reverse the court of appeals' judgment only if the trial court's erroneous exclusion of evidence "probably caused the rendition of an improper judgment." TEX. R. APP. P. 61.1 ; see also Gee , 765 S.W.2d at 396 . In application, a judgment may not be reversed unless the error "can be said to have contributed in a substantial way to bring about the adverse judgment." Lorusso v. Members Mut. Ins. Co. , 603 S.W.2d 818 , 819-20 (Tex. 1980) (citing King v. Skelly , 452 S.W.2d 691 , 696 (Tex. 1970) ). This rule is based on our understanding that "a litigant is not entitled to a perfect trial for, indeed, few trials are perfect." Id. at 819 . The determination of whether the error probably caused the rendition of an improper judgment "necessarily is a judgment call entrusted to the sound discretion and good senses of the reviewing court," McCraw v. Maris , 828 S.W.2d 756 , 759 (Tex. 1992), and we have recognized "the impossibility of establishing a specific test for determining harmful error." Caffe Ribs, Inc. , 487 S.W.3d at 145 (citing State v. Cent. Expressway Sign Assocs. , 302 S.W.3d 866 , 870 (Tex. 2009) ); see also Lorusso , 603 S.W.2d at 821 (citing Standard Fire Ins. Co. v. Reese , 584 S.W.2d 835 (Tex. 1979) ).

We have, however, noted that the role excluded evidence plays in the context of trial is important, and we have provided guidelines to assist trial courts in applying the reversible error standard. See Cent. Expressway Sign Assocs. , 302 S.W.3d at 870 . Exclusion is likely harmless if the evidence was cumulative or if the rest of the evidence was so one-sided that the error likely made no difference in the judgment. Id. By contrast, exclusion of the evidence is likely harmful if it was "crucial to a key issue." Id. We do not dispute that future medical expenses, which made up almost 70% of the trial court's judgment, was indeed a "key issue." But that does not end the inquiry; our guidelines make clear that even if the exclusion of evidence is crucial to a key issue, it is " likely harmful," not conclusively or per se harmful. See id. (emphasis added). And our guidelines do not displace the standard-otherwise the standard would become meaningless for entire categories of error. See In re D.I.B. , 988 S.W.2d 753 , 759 (Tex. 1999) ("[A]ppellate courts should not automatically foreclose the application of the harmless error test to certain categories of error." (quoting Cain v. State , 947 S.W.2d 262 , 264 (Tex. Crim. App. 1997) ) ); Walker v. Tex. Emp'rs' Ins. Ass'n , 155 Tex. 617 , 291 S.W.2d 298 , 301 (1956) ("Reversal may not be predicated upon a simple showing that error occurred and that the jury returned a verdict in some respects favorable to the party the error was reasonably calculated to help. If it could, the further provision of [the predecessor to Rule 61.1 ] that it must also appear that the error 'probably did cause the rendition of an improper judgment' would be meaningless and pointless."). Thus, in determining whether the exclusion of evidence was harmful, we must review the entire record, Tex. Dept. of Transp. v. Able , 35 S.W.3d 608 , 617 (Tex. 2000), and we apply the same standard-whether the erroneous exclusion of evidence probably caused the rendition of an improper judgment-even when the excluded evidence related to a key issue. Caffe Ribs, Inc. , 487 S.W.3d at 144-45 ; Gee , 765 S.W.2d at 396 .

The dissent manipulates the language we have used in analyzing whether error is harmful by omitting the word "likely" from our guidelines and introducing a two-part "framework" under which an error "is considered to have caused rendition of an improper judgment ... if the excluded testimony was crucial to a key issue, provided that the excluded evidence was not merely cumulative and that the other evidence was not so one-sided that the excluded evidence likely would have made no difference." 554 S.W.3d at 681. However, we have never endorsed this standard and have in fact noted that establishing a bright-line rule-as the dissent proposes to do-is impossible. Caffe Ribs, Inc. , 487 S.W.3d at 145 ; see also Lorusso , 603 S.W.2d at 821 ("We recognize the impossibility of prescribing a specific test for determining whether any error ... did cause the rendition of an improper judgment. Such a determination necessarily is a judgment call entrusted to the sound discretion and good senses of the reviewing court.") (internal quotations omitted). Our standard does not require that a complaining party show that evidence was not cumulative, not so one-sided, or even crucial to a key issue. "The complaining party must only show that the exclusion of evidence probably resulted in rendition of an improper judgment." Cent. Expressway Sign Assocs. , 302 S.W.3d at 869 (internal quotations omitted).

Dr. Gunn and OGA explained to the trial court that Dr. Schilling's video deposition testimony revolved around her line-item annotations to the life-care plan that Dr. Willingham had prepared, and in their offer of proof, they tendered Dr. Schilling's marked-up copy of Dr. Willingham's life-care plan for Shannon to be admitted into the record. 14 Dr. Gunn and OGA argued that the excluded video testimony indicated that Dr. Schilling believed some items in Dr. Willingham's life-care plan were not reasonably necessary, and that Dr. Schilling believed some items in that life-care plan related to Shannon's subsequent conditions (her seizure and hypoxia in 2005) and thus were unrelated to Dr. Gunn's alleged negligence.

Dr. Willingham's life-care plan projected future medical expenses for two care options, one for home care (Option 1) and one for facility-based care (Option 2). The two options were clearly presented to the jury. Both plans accounted for a twenty-year life expectancy. Dr. Willingham's testimony enumerated a number of categories of expenses-physician services, therapeutic services, medication, diagnostics, and supported life care, among others. He explained in detail what each category entailed, and he discussed why the projected expenses for some categories were the same for Option 1 and Option 2 while others were different for the two options. 15

He testified that, in total, the projected future medical expenses were "just over $6.9 million" for Option 1 and "just over $7.3 million" for Option 2. 16 He then explained that Dr. Schilling had made annotations to his life-care plan, "getting rid of some items and reducing the frequency of some items and lowering the cost of some items." He testified that implementing Dr. Schilling's changes and criticisms reduced the projected costs to $3.3 million for Option 1 and $6.7 million for Option 2.

Neither of the two witnesses' testimony purported to speak to liability, which the jury ultimately decided in favor of McCoy. Therefore, although the exclusion of Dr. Schilling's video testimony was error, we are satisfied that the exclusion did not affect the jury's determination of liability. Nor did it cause the rendition of an improper judgment with regard to awarding future medical expenses, since both experts conceded that Shannon would require several millions of dollars in future medical expenses. And the parties do not dispute that the amount of Shannon's future medical expenses would be at least equal to Dr. Schilling's reduced projections for Option 1 and Option 2-a minimum of $3.3 million. Thus, the only harm that could have resulted from the exclusion of Dr. Schilling's testimony was with respect to the amount of the award for future medical expenses that exceeded Dr. Schilling's minimum projection of $3.3 million.

Furthermore, neither Dr. Willingham's testimony nor Dr. Schilling's proffered video testimony attempted to persuade the jury that one option was preferable to the other. On the contrary, Dr. Willingham specifically stated on cross examination that he was not making a judgment call as to which option was preferable: "Either venue is appropriate, either care remaining within the home with caregivers brought in or moving to a life care residential. Either venue is appropriate." Likewise, Dr. Schilling's excluded deposition testimony contains nothing that might have suggested to the jury that home care would have been preferable. The jury was presented with ample testimony to understand the nature of the two options and chose Option 2, facility-based care. Thus, the question is what effect, if any, the erroneous exclusion of Dr. Schilling's video testimony had on the amount of damages awarded under the Option 2 plan, and whether the exclusion probably caused the jury to improperly award future medical expenses in excess of her projection for that option, $6.7 million. 17

Dr. Gunn and OGA do not challenge the legal sufficiency of the evidence supporting the jury's award of future medical expenses; therefore, that issue is not before us. We note that there is evidence to support Dr. Willingham's projected expenses, and the jury was presented with some evidence of Dr. Schilling's projections and criticisms. An award of future medical expenses is, by its very nature, not a matter of certainty. See, e.g. , Gulf, C. & S.F. Ry. Co. v. Harriett , 80 Tex. 73 , 15 S.W. 556 , 559 (1891) ("[T]he evidence should show that there is a reasonable probability of the occurrence of future ill effects of the injury, and [ ] it need show no more in order to justify the jury in considering future consequences in estimating the damages."); see also Rosenboom Mach. & Tool, Inc. v. Machala , 995 S.W.2d 817 , 828 (Tex. App.-Houston [1st Dist.] 1999, pet. denied) (explaining that, to recover future medical expenses, "the plaintiff must present evidence to establish that in all reasonable probability, future medical care will be required and the reasonable cost of that care"). The fact that additional evidence might have led the jury to award a different amount of future medical expenses does not mean that the jury's award without that evidence was unsupportable or improper. See Aultman v. Dallas Ry. & Terminal Co. , 152 Tex. 509 , 260 S.W.2d 596 , 600 (1953) (discussing the meaning of "probably" and recognizing that error can be harmless when it has no bearing on the issue of liability and could have influenced only the amount of damages, and the record contains ample support for the damages award). At the end of the day, we afford the jury discretion in awarding damages within the range of evidence presented at trial. Sw. Energy Prod. Co. v. Berry-Helfand , 491 S.W.3d 699 , 713 (Tex. 2016) (explaining that when determining whether the jury had a sufficient evidentiary foundation on which to base its damages award, we limit our review to only the evidence tending to support the jury's damages award, unless contrary evidence is conclusive).

An error in excluding evidence is harmful when it "probably" caused the rendition of an improper judgment, and "probably" is a higher standard than "might" or "could have." See Caffe Ribs, Inc. , 487 S.W.3d at 144-45 (stating that a trial court's exclusion of evidence "is reversible only if it probably caused the rendition of an improper judgment"); Aultman , 260 S.W.2d at 600 (defining "probably" as "having more evidence for than against; supported by evidence which inclines the mind to believe, but leaves some room for doubt; likely"). Of course, this standard does not require the complaining party to prove that " 'but for' the exclusion of evidence, a different judgment would necessarily have resulted." Cent. Expressway Sign Assocs. , 302 S.W.3d at 870 . But "likely" does not mean "definitively"-we must review the record to determine whether, in this particular case, the exclusion of Dr. Schilling's video deposition testimony "probably," as opposed to "might have" or "could have," caused the rendition of an improper judgment. Having reviewed the entire record, we cannot say that the erroneous exclusion of Dr. Schilling's video deposition testimony probably caused the rendition of an improper judgment. Dr. Gunn and OGA have not shown the trial court's error to be harmful, and we therefore may not reverse on this basis. TEX. R. APP. P. 61.1.

This is, of course, not to say that a plaintiff can effectively preempt a defense expert's presentation of evidence or a witness by having one of the plaintiff's expert witnesses discuss the substance of the defense witness's testimony. That cannot be the case. We hold only that in this case, on the record before us, the exclusion of Dr. Schilling's testimony did not "probably cause the rendition of an improper judgment." Id. ; see Caffe Ribs, Inc. , 487 S.W.3d at 144-45 ; Gee , 765 S.W.2d at 396 .

IV. Medical Billing Affidavits

As their second and third issues respectively, Dr. Gunn and OGA argue that McCoy failed to present legally sufficient evidence of Shannon's past medical expenses, for which the jury awarded $703,985.98. Specifically, they argue that the affidavits McCoy served did not comply with Texas Civil Practice and Remedies Code section 18.001 and therefore constituted no evidence, and in the absence of proper affidavits, McCoy failed to provide expert testimony regarding the reasonableness and necessity of the fees.

Section 18.001 governs proving expenses by affidavit. TEX. CIV. PRAC. & REM. CODE § 18.001. It is common to use section 18.001 affidavits as evidence of the reasonableness and necessity of past medical expenses. See, e.g. , Haygood v. De Escabedo , 356 S.W.3d 390 , 397-99 (Tex. 2011). The statute provides:

Unless a controverting affidavit is served as provided by this section, an affidavit that the amount a person charged for a service was reasonable at the time and place that the service was provided and that the service was necessary is sufficient evidence to support a finding of fact by judge or jury that the amount charged was reasonable or that the service was necessary.

TEX. CIV. PRAC. & REM. CODE § 18.001(b). To comply with this section, an affidavit must be made by "(A) the person who provided the service; or (B) the person in charge of records showing the service provided and the charge made." Id. § 18.001(c)(2). Consistent with other parts of the Code, the amount listed on the affidavit is limited to the amount actually paid or incurred, not the amount billed. Haygood , 356 S.W.3d at 390 , 398 (citing TEX. CIV. PRAC. & REM. CODE § 41.0105 ). Generally speaking, section 18.001 is "purely procedural, providing for the use of affidavits to streamline proof of the reasonableness and necessity of medical expenses." Id. at 397 . Thus, the affidavits are not conclusive; the statute expressly provides that they can be controverted by competing affidavit. TEX. CIV. PRAC. & REM. CODE § 18.001(b) ; Haygood , 356 S.W.3d at 397-98 .

McCoy initially served fourteen section 18.001 affidavits from Shannon's medical providers or those providers' record custodians, containing the billed amounts. After this Court issued its opinion in Haygood , McCoy withdrew the provider affidavits and filed affidavits from subrogation agents for health insurance carriers that had paid Shannon's medical expenses, reflecting instead the amounts actually paid. Dr. Gunn and OGA objected, arguing that section 18.001 limits the proper affiants to providers or record custodians for those providers. We disagree.

Our primary goal in statutory construction is to give effect to the Legislature's intent. Tex. Lottery Comm'n v. First State Bank of DeQueen , 325 S.W.3d 628 , 635 (Tex. 2010). We rely on the plain meaning of the text as expressing legislative intent unless a different meaning is supplied by legislative definition or is apparent from the context, or the plain meaning leads to absurd results. Id. We have already acknowledged that the Legislature's intent in enacting section 18.001 was "to streamline proof of the reasonableness and necessity of medical expenses." Haygood , 356 S.W.3d at 397 . We further note that the plain language of section 18.001(c)(2)(B) does not require that affidavits be made by a records custodian for a medical provider . TEX. CIV. PRAC. & REM. CODE § 18.001(c)(2)(B) ; see also Tex. Mut. Ins. Co. v. Ruttiger , 381 S.W.3d 430 , 452 (Tex. 2012) ("[T]his Court presumes the Legislature deliberately and purposefully selects words and phrases it enacts, as well as deliberately and purposefully omits words and phrases it does not enact."). Still, we recognize that an effort to "streamline" proof of the reasonableness and necessity of medical expenses cannot negate the requirement that reasonableness and necessity be in fact proven by legally sufficient evidence. See Haygood , 356 S.W.3d at 397-98 . We therefore assess whether subrogation agents are in a position to testify to the reasonableness and necessity of medical expenses.

Health care costs today are complex, and the price of a particular provider's services may depend on many factors, including geography, experience, location, government payment methods, and the desire to make a profit. Keith T. Peters, What Have We Here? The Need for Transparent Pricing and Quality Information in Health Care: Creation of an SEC for Health Care , 10 J. HEALTH CARE L. & POL'Y 363, 366 (2007). Hospitals have developed a two-tier pricing system: (1) the "list price," which serves as a starting point for negotiations, similar to the sticker price one might find when purchasing a vehicle; and (2) the "actual price," which is what private insurers, Medicaid and Medicare, and other groups actually pay after negotiations. Id. ; Haygood , 356 S.W.3d at 393 . While hospital prices ideally should be determined by the cost of providing care, in reality list prices are driven in large part by the reimbursement rates for federal programs such as Medicare and Medicaid, and by negotiations with private insurers. Haygood , 356 S.W.3d at 393 (citing Peters, supra at 366 ).

Although the list price of health care varies widely across different regions of the country, the actual price paid is relatively static. Peters, supra at 366. Thus, it is not uncommon or surprising that a given medical provider may have no basis for knowing what is a "reasonable" fee for a specific service. See id. at 364 (discussing the lack of transparency in health care pricing and detailing a personal account in which the business office of a hospital "did not know and had no list of typical charges" for the cost of delivering a child, a routine procedure). While hospitals may devote significant time and effort to establishing and updating their list prices, they generally establish those prices with the clear expectation that they will be paid only a portion of them. George A. Nation III, Obscene Contracts: The Doctrine of Unconscionability and Hospital Billing of the Uninsured , 94 KY. L.J. 101, 118 (2005). Thus, for most consumers today, the most important function of a health insurance company in a given year is not paying a portion of health care expenses, but negotiating the prices for those services with providers. Jackson Williams, The Persistence of Opportunistic Business Models in Health Care and a Stronger Role for Insurance Regulators in Containing Health Care Costs , 41 NOVA L. REV. 313, 315 (2017).

We have previously noted that agreements between willing providers and willing insurers can yield reasonable rates. See Haygood , 356 S.W.3d at 393-94 . Insurance companies, which insure consumers across the nation, regularly negotiate with providers to agree upon the actual prices. See id. at 393 ; Williams, supra at 315 . Moreover, insurance companies keep records and databases of both the list prices and the actual prices of specific treatments and procedures, though they may not pass this information to consumers. See Peters, supra at 375 (noting as examples that Blue Cross and Blue Shield of North Carolina provides the low, high, and average retail prices of specific procedures on its website, and that Aetna initiated a controversial pilot program in 2005 making information on the actual price for common procedures available to its insureds). Thus, with national and regional bases on which to compare prices actually paid, insurance agents are generally well-suited to determine the reasonableness of medical expenses.

In determining the necessity of medical expenses, it is beyond argument that medical providers-doctors-are in the best position to determine what treatments or procedures and resulting expenses are "necessary." So of course, ideally, medical providers themselves would testify to the necessity of medical expenses. However, by drafting section 18.001 to allow either "the person who provided the service" or the "person in charge of records" to testify to reasonableness and necessity, the Legislature has acknowledged and made allowance for the reality that the ideal paradigm does not reflect today's complex health care system. And, for better or for worse, in the context of our health care system, what is "necessary" is often heavily influenced by insurance companies and by what treatments and procedures they are willing to cover. Janet L. Dolgin, Unhealthy Determinations: Controlling "Medical Necessity," 22 VA. J. SOC. POL'Y & L. 435, 442-43 (2015) (explaining that the insurance industry sits at the center of the delivery and coverage of health care in the United States and occupies "a privileged position in rendering medical necessity determinations-the rationale in terms of which health care is apportioned").

Thus, the plain language of section 18.001 does not limit the proper affiants to medical providers and medical providers' record custodians, and the reality of our health care system does not mandate such a limitation in order to establish the reasonableness and necessity of expenses. By contrast, the dissent's argument would read into the statute that an affidavit must be made by a records custodian "sufficiently trained or experienced in medicine to give competent testimony as to the necessity of treatment and sufficiently familiar with reasonableness of charges in the region or locale in which services were rendered to give competent testimony as to the reasonableness of amounts paid by insurance companies," as well requiring that an affiant be "reasonably connected to the patient" or have "first-hand knowledge of the services rendered." But see TGS-NOPEC Geophysical Co. v. Combs , 340 S.W.3d 432 , 439 (Tex. 2011) ("We presume that the Legislature chooses a statute's language with care, including each word chosen for a purpose, while purposefully omitting words not chosen."). These heightened requirements do not appear in the language of the statute, nor do they comport with the Legislature's intent to streamline proof of the reasonableness and necessity of medical expenses. See Haygood , 356 S.W.3d at 397 .

We note that in this case, McCoy did produce affidavits from the medical providers or the providers' records custodians, which were later rendered insufficient by our decision in Haygood because they reflected only the amount billed. See 356 S.W.3d at 398-99 . If we were to hold that the subrogation agents' affidavits were also insufficient, as the dissent advocates and Dr. Gunn and OGA would have us do, we would in effect render the Legislature's streamlined proof procedure a complicated trap requiring plaintiffs to provide two sets of affidavits: (1) affidavits as to necessity of treatment from medical providers with actual knowledge of the patient's treatment, or their record custodians; and (2) separate affidavits as to reasonableness of paid charges from local or regional insurance agents or someone else with knowledge of customary amounts paid for particular treatments in that particular region. Such an approach would frustrate the Legislature's intent. We reiterate that an affidavit served under section 18.001 is "purely procedural" and does not amount to conclusive evidence of the expenses. See TEX. CIV. PRAC. & REM. CODE § 18.001 ; Haygood , 356 S.W.3d at 397-98 . The statute "expressly contemplates that the issue can be controverted by affidavit." Haygood , 356 S.W.3d at 397 ; accord. TEX. CIV. PRAC. & REM. CODE § 18.001. We hold that the affidavits served by McCoy were proper and constituted legally sufficient evidence of the reasonableness and necessity of Shannon's past medical fees.

V. Jury Instruction

At trial, Dr. Gunn and OGA requested the following instruction, which the trial court refused: "An occurrence may be an 'unavoidable accident,' that is, an event not proximately caused by the negligence of any party to the occurrence." They now argue that the trial court committed harmful error by refusing to include their requested instruction in the jury charge. A trial court has considerable discretion to determine proper jury instructions, and we review a trial court's decision to submit or refuse a particular instruction for an abuse of discretion. Thota v. Young , 366 S.W.3d 678 , 687 (Tex. 2012). One way in which a trial court may abuse its discretion is by failing to follow guiding rules and principles. Hawley , 284 S.W.3d at 856 . An instruction is proper if it assists the jury, accurately states the law, and finds support in the pleadings and evidence. Id. at 855 . Still, we do not reverse a judgment based on charge error unless the error probably caused the rendition of an improper judgment or prevented the petitioner from properly presenting the case to the appellate courts. TEX. R. APP. P. 61.1. Thus, when a trial court refuses to submit a requested instruction that is otherwise proper, the question on appeal is whether the request was reasonably necessary to enable the jury to render a proper verdict. Shupe v. Lingafelter , 192 S.W.3d 577 , 579 (Tex. 2006) (per curiam) (citing Tex. Workers' Comp. Ins. Fund v. Mandlbauer , 34 S.W.3d 909 , 912 (Tex. 2000) ).

An unavoidable accident is "an event not proximately caused by the negligence of any party to it." Reinhart v. Young , 906 S.W.2d 471 , 472 (Tex. 1995). The purpose of the unavoidable-accident instruction is to advise the jurors that "they do not have to place blame on a party to the suit if the evidence shows that conditions beyond the party's control caused the accident." Dillard v. Tex. Elec. Coop. , 157 S.W.3d 429 , 432 (Tex. 2005) (citing Reinhart , 906 S.W.2d at 472 ). An instruction on unavoidable accident is "most often used to inquire about the causal effect of some physical condition or circumstance such as fog, snow, sleet, wet or slick pavement, or obstruction of view, or to resolve a case involving a very young child who is legally incapable of negligence." Reinhart , 906 S.W.2d at 472 . But the instruction is not limited to only those circumstances-it merely informs the jury that it may consider causes of the occurrence other than the negligence of the parties. Dillard, 157 S.W.3d at 433 . In any case, such an instruction is proper only when there is evidence that the event was proximately caused by a nonhuman condition and not by the negligence of any party to the event. Hill v. Winn Dixie Tex., Inc. , 849 S.W.2d 802 , 803 (Tex. 1992).

The parties agree that Shannon presented at Woman's Hospital with placental abruption and DIC and that those conditions were not the fault of any party. Dr. Gunn and OGA argue that Dr. Aubuchon and Dr. Steiner both connected Shannon's brain injury to a non-negligent cause-namely, DIC-induced microthrombi or "little floating clots" that lodged in the small vessels in Shannon's brain and deprived it of blood and the oxygen it carries. Therefore, they argue, the unavoidable-accident instruction would have assisted the jury. See Dillard , 157 S.W.3d at 433 ("There is at least a potential implication in [the broad-form jury question] that the occurrence was caused by someone's negligence. We see no harm in explaining to the jury through an inferential rebuttal instruction that no such implication is intended.") (internal citations omitted).

Indeed, the evidence of causation has been the subject of dispute throughout the pendency of this case. So we have little hesitation in determining that the requested instruction would have been proper. 18 See Hawley , 284 S.W.3d at 855 ("An instruction is proper if it (1) assists the jury, (2) accurately states the law, and (3) finds support in the pleadings and evidence."). "The truth is, sometimes accidents are no one's fault, and an unavoidable accident instruction ... simply explains to the jury that they are not required to find someone at fault." Bed, Bath & Beyond, Inc. v. Urista , 211 S.W.3d 753 , 757 (Tex. 2006). Be that as it may, however, the decision to refuse a requested instruction remains within the trial court's discretion, and a determination that a requested instruction was proper does not render it mandatory. Thota , 366 S.W.3d at 687 . We, like the court of appeals, find no authority supporting the conclusion that the trial court in this case erred in refusing to submit an unavoidable-accident instruction. See 489 S.W.3d at 115 ; see also Towers of Town Lake Condo. Ass'n v. Rouhani , 296 S.W.3d 290 , 301 (Tex. App.-Austin 2009, pet. denied) ("Significantly, though, while the supreme court has held that the trial court may, in its discretion, submit the instruction under such circumstances, it has not held that it is an abuse of discretion not to do so.").

Even assuming the trial court abused its discretion in refusing to submit a jury instruction on unavoidable accident, we conclude that Dr. Gunn and OGA failed to show that the omission probably caused the rendition of an improper judgment. 19 When a trial court errs in refusing to submit an otherwise proper instruction, the question on appeal is whether the request was reasonably necessary to enable the jury to render a proper verdict. Shupe , 192 S.W.3d at 579 . To determine whether the instruction, or the omission of one, probably caused an improper judgment, we examine the entire record. See Urista , 211 S.W.3d at 757 . We have already discussed both causation theories in depth.

Provided with these alternative theories, the jury was free to determine which to credit. Mel Acres Ranch , 443 S.W.3d at 833 ; City of Keller , 168 S.W.3d at 819 ("Jurors are the sole judges of the credibility of the witnesses and the weight to give their testimony."). Therefore, we assume the jury decided all credibility issues in favor of the verdict if reasonable human beings could do so. City of Keller , 168 S.W.3d at 819 . In light of the conflicting expert testimony and because a jury could have reasonably believed Dr. Brewer's blood-loss theory of causation, we conclude that an unavoidable-accident instruction would have merely reiterated the defense's causation theory and therefore was not necessary for the jury to render a proper verdict. See Thota , 366 S.W.3d at 695-96 (holding that the trial court's submission of the new and independent cause instruction was harmless error because conflicting expert testimony could have prompted a reasonable jury not to find the defendant negligent, regardless of its conclusion on the question of proximate cause).

VI. Summary Judgment on Comparative Responsibility

As her fourth issue, Dr. Gunn asserts that the trial court erred in granting McCoy's no-evidence summary judgment motion as to the defendants' affirmative defense of comparative responsibility. However, while the issue was raised in the "Issues Presented" section in both the petition for review and the brief on the merits, Dr. Gunn failed to support her contention with any argument or authority in either the petition or the brief. Every issue presented by a party must be supported by argument and authorities in the party's brief on the merits, or it is waived. Trenholm v. Ratcliff , 646 S.W.2d 927 , 934 (Tex. 1983) ; see also TEX. R. APP. P. 55.2(i) ("The brief must contain a clear and concise argument for the contentions made, with appropriate citations to authorities and to the record."). Thus, Dr. Gunn's issue of comparative responsibility is waived.

VII. OGA's Indemnity Claims

As her fifth issue, Dr. Gunn argues that OGA's indemnity claim will not be ripe unless and until there is a final payable judgment at the conclusion of the appeal. Essentially, she argues that OGA may not pursue inconsistent positions-that Dr. Gunn was not negligent but that OGA is entitled to indemnity from her-simultaneously after the verdict. Generally speaking, "a person who, without personal fault, has become subject to tort liability for the unauthorized and wrongful conduct of another, is entitled to full indemnity from the other for expenditures properly made to discharge the liability." SSP Partners v. Gladstrong Invs. (USA) Corp. , 275 S.W.3d 444 , 457 (Tex. 2008) (emphasis removed) (quoting Humana Hosp. Corp. v. Am. Med. Sys., Inc. , 785 S.W.2d 144 , 145 (Tex. 1990) ). Of course, we have held that the comparative-negligence statute 20 effectively abolished the common law doctrine of indemnity between joint tortfeasors. Aviation Office of Am., Inc. v. Alexander & Alexander of Tex., Inc. , 751 S.W.2d 179 , 180 (Tex. 1988). Thus, "[t]he only remaining vestiges of common law indemnity involve purely vicarious liability or the innocent product retailer situation." Id. OGA's liability in this case is purely vicarious, and thus it is entitled to bring a cause of action for indemnity against Dr. Gunn.

A cause of action for indemnity accrues when "the indemnitee's liability to the party seeking damages becomes fixed and certain, generally by a judgment." Am. Star Energy & Minerals Corp. v. Stowers , 457 S.W.3d 427 , 432-33 (Tex. 2015) (citing Ingersoll-Rand Co. v. Valero Energy Corp. , 997 S.W.2d 203 , 208 (Tex. 1999) (internal quotations omitted) ). However, "an indemnitee may bring a claim against an indemnitor before the judgment is assigned against the indemnitee"-before the cause of action accrues and before limitations begin to run. Id. Thus, our law does not require that a judgment be fixed and payable on appeal in order to sustain a claim for indemnity. See, e.g. , SSP Partners , 275 S.W.3d at 457 ("[I]t is a general principle of law that an active wrongdoer may be made to indemnify one who has been subjected to, or is sought to be held liable for , damage through his wrong ." (second emphasis added) ).

The court of appeals correctly noted that "[Dr.] Gunn has not provided us with, and we have not located, any authority indicating that an indemnity claim only ripens when any related liability appeal is completed." 489 S.W.3d at 117 . Instead, Dr. Gunn argues, "If a party seeks inconsistent theories of defense, Texas law does not require it to elect between those theories before verdict, but a party is not entitled to maintain inconsistent positions after the verdict." She cites International Piping Systems, Ltd. v. M.M. White & Associates, Inc. , to support this proposition. 831 S.W.2d 444 , 452 (Tex. App.-Houston [14th Dist.] 1992, writ denied) ("If a party pleads more than one theory of recovery or defense, normally he need not elect between them until after the verdict."), disapproved of by Great Am. Ins. Co. v. N. Austin Mun. Util. Dist. No. 1 , 950 S.W.2d 371 (Tex. 1997) (per curiam). We fail to see how this case supports Dr. Gunn's contention. Even if the case were persuasive, the previous sentence states that "our liberal system of pleading allows a party to state his claims or defenses alternatively or hypothetically and to state as many claims as he has regardless of consistency." Id. This is consistent with the Texas Rules of Civil Procedure, which permit a party to set forth alternative claims of defense and says nothing about whether those positions are maintained while a case is pending appeal. TEX. R. CIV. P. 48. We conclude that OGA's common law indemnity claim was ripe for determination when the trial court rendered its judgment against Dr. Gunn notwithstanding the pending appeal.

VIII. Windfall

As her sixth issue, Dr. Gunn argues that Shannon's death on the eve of the court of appeals' decision created a windfall for McCoy that calls for a remand in the interest of justice. She notes that almost 70% of the trial court's judgment was for future medical expenses, which will now never be incurred. Texas Rule of Appellate Procedure 7.1 speaks to this very situation:

If a party to a civil case dies after the trial court renders judgment but before the case has been finally disposed of on appeal, the appeal may be perfected, and the appellate court will proceed to adjudicate the appeal as if all parties were alive. The appellate court's judgment will have the same force and effect as if rendered when all parties were living.

TEX. R. APP. P. 7.1. We do not disagree that if this case were remanded to the trial court for a new trial, the issue of future medical expenses would become moot (and the amount of past medical expenses would almost certainly increase); however, that is not the disposition of this case. Cf. In re Dep't. of Family & Protective Servs. , 273 S.W.3d 637 , 644 (Tex. 2009) (orig. proceeding) ("When a trial court grants a motion for new trial, the case is reinstated on the trial court's docket as though no trial had occurred, and the slate is essentially wiped clean."). We have already determined that liability in this case was established by legally sufficient evidence and that future medical expenses were proven to an ascertainable amount with reasonable certainty. Therefore, we issue our judgment as if Shannon were still alive, as Rule 7.1 requires.

We note that the outcome may well be different had the trial court awarded McCoy periodic payments of future medical expenses. Texas Civil Practice and Remedies Code section 74.506 provides:

(a) On the death of the recipient, money damages awarded for loss of future earnings continue to be paid to the estate of the recipient of the award without reduction.

(b) Periodic payments, other than future loss of earnings, terminate on the death of the recipient.

TEX. CIV. PRAC. & REM. CODE § 74.506. When requested by a defendant physician or health care provider, a trial court must order that medical, health care, or custodial services awarded in a health care liability claim be paid in whole or in part in periodic payments. Id. § 74.503(a). When periodic payments are ordered, the court must make specific findings as to the amount of periodic payments, and the court's judgment must specify the amount, the timing of payments, and the number of payments or time period over which payments are to be made. Id. § 74.503(c), (d). The trial court's judgment in this case awarded McCoy future medical expenses as a lump-sum payment, and there is nothing in the record to indicate that Dr. Gunn or OGA requested periodic payments. Had the future medical expenses been awarded as periodic payments, any remaining payments would have terminated upon Shannon's death. See St. Joseph Reg'l Health Ctr. v. Hopkins , 393 S.W.3d 885 , 886 (Tex. App.-Waco 2012, pet. denied) (recognizing that the death of a recipient of periodic payments of future medical expenses would terminate such payments). But pursuant to Rule 7.1, we review and adjudicate the lump-sum award of future medical expenses as if Shannon were still alive.

Besides that, we are hardly persuaded by the argument that McCoy received a windfall merely because Shannon's actual life span was shorter than her projected life span. While evidence must establish a reasonable probability of future medical expenses in order to support an award, such an award, by its nature, evades certainty. See Harriett , 15 S.W. at 559 ; Rosenboom Mach. & Tool, Inc. , 995 S.W.2d at 828 . Dr. Willingham projected and Dr. Schilling did not contest that Shannon's future medical expenses were based on a twenty-year life expectancy. And Shannon could have outlived that expectancy, in which case the judgment would have been for less than she needed. Further, based on Dr. Willingham's testimony, the jury clearly considered the possibility that Shannon's life expectancy would end up not being accurate:

Q: So if she lives for one year longer than you're forecasting, she is going to run out of money, isn't she?

A: And this would understate her need, that's correct.

But of course, we would not go back and adjust the judgment upwards if she had outlived Dr. Willingham's projection. Similarly, we do not vacate a judgment for future medical expenses simply because a party died earlier than projected. Cf. Gibbs v. Belcher , 30 Tex. 79 , 84-85 (Tex. 1867) (concluding that the death of the party after judgment did not have the effect of vacating or opening the judgment). We decline to remand this case based solely on Shannon's premature death.

IX. Conclusion

We hold that: (1) the evidence is legally sufficient to support causation; (2) although the trial court erred in excluding the video deposition testimony of Dr. Schilling, the exclusion did not probably cause the rendition of an improper verdict; (3) Texas Civil Practice and Remedies Code section 18.001 does not limit the proper affiants to medical providers and medical providers' record custodians; therefore, the affidavits served by McCoy were proper and constituted legally sufficient evidence of the reasonableness and necessity of Shannon's past medical expenses; (4) Dr. Gunn and OGA failed to show that the trial court committed harmful error in refusing the requested instruction on unavoidable accident; (5) Dr. Gunn waived her challenge to the no-evidence summary judgment on comparative responsibility; (6) OGA's indemnity claim was ripe when the trial court rendered its judgment against Dr. Gunn; and (7) Shannon's death does not mandate a remand of the case. For the reasons expressed, we affirm the judgment of the court of appeals in its entirety.

Justice Johnson filed a dissenting opinion, in which Justice Boyd joined.

Justice Guzman did not participate in the decision.

Justice Johnson, joined by Justice Boyd, dissenting.

I disagree that the trial court's error in excluding the defendants' expert witness testimony regarding Shannon McCoy's future medical care expenses was harmless.

I also disagree that affidavits by subrogation agents with no disclosed familiarity or connection with the medical treatment in question provide sufficient basis to either admit the summaries of medical expenses attached to them, or support jury findings as to the reasonable and necessary amounts for past medical expenses.

Further, even if the subrogation agents' affidavits would have otherwise provided a sufficient basis to admit the attached documents and support jury findings as to medical expenses, I disagree that all of the affidavits support the jury findings as to past medical expenses. One of the affidavits referencing over $300,000 of expenses was not sufficient because it did not address whether the expenses in the attached exhibits were necessary.

I. Future Medical Expenses

At trial, the parties did not dispute that Shannon's conditions would require medical care for the rest of her life. But they disputed both whether all her conditions were proximately caused by the allegedly negligent actions of the defendants, Obstetrical and Gynecological Associates, P.A. (OGA) and Dr. Debra Gunn, as well as the cost of future care that would be medically necessary as a result of those actions. Shannon's husband, Andre McCoy (McCoy), suing individually and as Shannon's guardian, offered, and the trial court admitted, opinion testimony of Dr. Alex Willingham regarding the probable costs of Shannon's future care. The life-care plan he prepared was introduced in connection with his testimony. To controvert Dr. Willingham's testimony, OGA and Dr. Gunn sought to introduce video deposition testimony from Dr. Hellen Schilling. I agree with the Court that the exclusion of Dr. Schilling's testimony was error. Ante at ----. However, I disagree that the error was harmless.

The standard for reversal of a judgment based on the exclusion of evidence is whether the error "probably caused the rendition of an improper judgment." TEX. R. APP. P. 61.1 ; see Gee v. Liberty Mut. Fire Ins. Co. , 765 S.W.2d 394 , 396 (Tex. 1989). We have explained that "[i]t is not necessary for the complaining party to prove that 'but for' the exclusion of evidence, a different judgment would necessarily have resulted." State v. Cent. Expressway Sign Assocs. , 302 S.W.3d 866 , 870 (Tex. 2009) (quoting McCraw v. Maris , 828 S.W.2d 756 , 758 (Tex. 1992) ). Rather, we have explicated a framework for distinguishing between a harmless exclusion of evidence, one that did not probably cause rendition of an improper judgment, and a harmful exclusion, one that probably did cause rendition of an improper judgment. See id. If erroneously excluded evidence was "crucial to a key issue," then the error was likely harmful; that is, it probably caused the rendition of an improper judgment. Id. ; see also Diamond Offshore Servs. Ltd. v. Williams , 542 S.W.3d 539 , 551 (Tex. 2018) ("[E]xclusion is likely harmful if the evidence is crucial to a key issue."); Caffe Ribs, Inc. v. State , 487 S.W.3d 137 , 145 (Tex. 2016) ; JLG Trucking, LLC v. Garza , 466 S.W.3d 157 , 165 (Tex. 2015) ; Reliance Steel & Aluminum Co. v. Sevcik , 267 S.W.3d 867 , 873 (Tex. 2008). On the other hand, that does not end the inquiry. Because even if the excluded evidence was crucial to a key issue, "the exclusion ... is likely harmless if the evidence was cumulative, or the rest of the evidence was so one-sided that the error likely made no difference in the judgment." Cent. Expressway Sign Assocs. , 302 S.W.3d at 870 (citing Reliance Steel & Aluminum Co. , 267 S.W.3d at 873 ).

Given the extent to which parties engage in discovery in cases such as this, there is very little, if any, evidence presented at trial that the parties' lawyers are unaware of before trial. Yet cases are tried because there is no way of accurately predicting how the evidence, or any particular piece of evidence, will affect a jury. Likewise, there is no realistic way of accurately determining how excluded evidence would have affected the jury's findings if it had been admitted. Thus, the conclusion that an error in excluding evidence relevant to a key issue "likely was harmful" is as close as any analysis is going to get. Stated another way, the test this Court has articulated is, in substance, that an error in excluding testimony is considered to have caused rendition of an improper judgment (the exclusion was likely harmful) if the excluded testimony was crucial to a key issue, provided that the excluded evidence was not merely cumulative and that the other evidence was not so one-sided that the excluded evidence likely would have made no difference. That is a reasonable and at least somewhat objective test that requires examination of the entire record. Because OGA and Dr. Gunn are the parties asserting error, the burden is on them to meet the test. Which, as explained below, in my view they did by demonstrating that the record shows: (1) the excluded evidence was crucial to what even McCoy's attorney told the jury was the most important issue; (2) the excluded evidence conflicted with the testimony of Dr. Willingham as to the amount of treatment and care that would be necessary for Shannon in the future, thus Dr. Schilling's testimony was not merely cumulative in regard to the key issue of the cost of necessary future medical care; and (3) the opinion testimony of Dr. Willingham was not so one-sided that Dr. Schilling's testimony likely would have had no effect at all on the jury's finding for future medical expenses. After all, Dr. Willingham's testimony was opinion testimony predicting the future, not factual testimony relating past events; Dr. Schilling and Dr. Willingham had substantively equivalent credentials; each examined Shannon one time for approximately the same amount of time; and each had extensive experience in preparing life-care plans for predicting the necessity of and costs for future care for severely impaired patients.

The Court subtly but importantly departs from the workable legal standard we have established, as is set out above, for determining whether the exclusion of evidence on a crucial issue was harmful. The Court opts for merely balancing the evidence admitted and excluded to determine whether, in its opinion the excluded evidence would have made a difference in the sitting jurors' minds and findings. The Court says,

We have, however, noted that the role excluded evidence plays in the context of trial is important, and we have provided guidelines to assist trial courts in applying the harmful error standard. Exclusion is likely harmless if the evidence was cumulative or if the rest of the evidence was so one-sided that the error likely made no difference in the judgment. By contrast, exclusion of the evidence is likely harmful if it was "crucial to a key issue." We do not dispute that future medical expenses, which made up almost 70% of the trial court's judgment, was indeed a "key issue." But that does not end the inquiry; our guidelines make clear that the exclusion of such evidence is " likely harmful," not conclusively or per se harmful.

Ante at 668 (citations omitted). The Court then, however, proceeds to briefly analyze the evidence and set out the standard it uses to make its decision:

An error in excluding evidence is harmful when it "probably" caused the rendition of an improper judgment, and "probably" is a higher standard than "might" or "could have." See Caffe Ribs, Inc. , 487 S.W.3d at 144-45 (stating that a trial court's exclusion of evidence "is reversible only if it probably caused the rendition of an improper judgment"); Aultman v. Dall. Ry. & Terminal Co. , 152 Tex. 509 , 260 S.W.2d 596 , 600 (1953) (defining "probably" as "having more evidence for than against, supported by evidence which inclines the mind to believe, but leaves some room for doubt, likely") ... But "likely" does not mean "definitively"- we must review the record to determine whether, in this particular case, the exclusion of Dr. Schilling's video deposition testimony "probably," as opposed to "might have" or "could have," caused the rendition of an improper judgment.

Ante at 671 (emphasis added). The Court's application of the standard ignores the fact that the effect of particular evidence on a particular jury-whether admitted or excluded-is just not reasonably predictable to a fine degree. And the questions of whether the excluded evidence was cumulative and whether it was so one-sided that the error likely made no difference as part of the analysis disappear for all practical purposes in the Court's analysis. But they should be the standards for evaluating the effect of the error after it has been determined that the error was likely harmful because the excluded evidence was crucial to a key issue. In situations such as this where the evidence was crucial to a key issue and was not cumulative, the harm analysis should turn on whether the evidence was so one-sided that the excluded evidence likely made no difference. The "likely" is tethered to the "so one-sided" part of the standard. Not so in the standard the Court applies today. And while the standard of "so one-sided" is to a degree subjective, it is not nearly so subjective as determining whether excluded evidence would "more likely than not" have caused the jury to return a different verdict, which in effect is merely a second-guessing of the jury's view of the evidence.

The Court's harm analysis in this case demonstrates the difference. After determining that the exclusion of Dr. Schilling's testimony was likely harmful because of its relationship to the key issue of future medical expenses, the Court moves directly to balancing the evidence. That necessarily means it considered how the jurors' thought processes likely would have worked and what the jury likely would have done had it heard the excluded evidence. Explaining a framework for that exercise is hard to do, as witnessed by the Court's explanation-less conclusion:

Having reviewed the entire record, we cannot say that the erroneous exclusion of Dr. Schilling's video deposition testimony probably caused the rendition of an improper judgment.

Ante at 671.

The Court also says that there's no bright line for deciding the harm question. I agree. But there must be some articulable standard which differs from merely balancing the evidence admitted and excluded. The standard we have previously articulated, properly applied, is such a standard.

Measuring the effect of the exclusion of Dr. Schilling's testimony once future medical was determined to be a key issue under the previously articulated standard-the exclusion was likely harmless if the excluded evidence was cumulative or the rest of the evidence was so one-sided that the error likely made no difference in the judgment, Cent. Expressway Sign Assocs. , 302 S.W.3d at 870 -it seems to me that, indeed, (1) the trial court erred by excluding Dr. Schilling's testimony; (2) her testimony was crucial to a key issue; (3) the testimony was not merely cumulative; and (4) the other evidence was not so one-sided that her testimony likely would have made no difference. Thus, the exclusion was not harmless.

Giving context to evidence determinative of whether the evidence was so one-sided that Dr. Schilling's testimony likely would have made no difference requires examining all the relevant evidence, beginning with whether the testimony of Dr. Schilling concerned a key issue. The starting point for analyzing that question is the judgment. Reliance Steel & Aluminum Co. , 267 S.W.3d at 871 .

First, that Shannon's future medical care expenses were a key issue is clear from the fact that of the jury's damage findings for over $10.6 million, over $7.2 million was for future medical care. The amount found for future medical care was more than double the nearly $3.4 million for other damage findings combined. Second, whether the future medical was a key issue was answered by McCoy's attorney in his closing summation to the jury. He stated that the last blank, where the jury was to place its finding as to Shannon's future medical care expenses, "is the most important blank."

The court of appeals opined that the key dispute was liability, not damages. 489 S.W.3d 75 , 112 (Tex. App.-Houston [14th Dist.] 2016). Unquestionably, the defendants consistently contested liability throughout the trial and in the court of appeals. They still do in this Court. But they also have consistently contested the amount of damages. We have never said that there is or can be only one key dispute in a trial. Nor does such a concept make sense. Many, if not the great majority of, cases involve heated contests over both liability and damages. Although it hardly needs to be said that there usually are multiple key issues in a trial, we specifically acknowledged that there are in Central Expressway , where we said that evidence either admitted in error or excluded in error was likely harmful if the evidence "was crucial to a key issue." 302 S.W.3d at 870 (emphasis added); see also Diamond Offshore Servs. Ltd. , 542 S.W.3d at 551 (stating exclusion is likely harmful if the evidence "is crucial to a key issue") (emphasis added); Caffe Ribs, Inc. , 487 S.W.3d at 145 (same); JLG Trucking, LLC , 466 S.W.3d at 165 (same).

Next, the harm analysis looks to whether Dr. Schilling's testimony was crucial to the key issue of future medical expenses for Shannon. And the answer is "Yes."

Lay jurors are deemed to lack the knowledge to determine the reasonable costs of medical care that was necessary in the past or will probably be necessary in the future. See Watkins v. Watkins , 2000 WL 1862686 , at *4 (Tex. App.-Austin 2000, pet. denied) (stating that a party cannot recover medical expenses without proof of what costs are reasonable and necessary (citing Dall. Ry. & Terminal Co. v. Gossett , 156 Tex. 252 , 294 S.W.2d 377 , 382-83 (1956) ) ). Thus, jurors must be guided by expert testimony in making their findings. See id. And almost categorically, both plaintiffs and defendants present-or at least try to present-experts to opine on the necessity of particular items of medical care and the cost of those items in the future, even when it is uncontested that future care will be necessary. And when experts testify, one aspect of persuading the jury to the presenting party's position is presenting the expert's qualifications. That is because it is generally accepted among experienced trial lawyers that (1) part of the jury's decision regarding expert testimony will turn on whose expert the jury believes is the more credible witness, and (2) a significant part of an expert's credibility relates to the expert's qualifications and experience as to the particular issue.

As the Court notes, whether sufficient evidence supports the jury's finding as to expenses for future medical care is not at issue. Ante at ----. Dr. Willingham's testimony is evidence to support the finding. But where Dr. Schilling was the defendants' only witness as to the question of future medical care expenses, her testimony was crucial to the question of the differences between her opinions and those of Dr. Willingham. That conclusion is buttressed by McCoy's attempts to keep Dr. Schilling's testimony out of evidence. Further, Dr. Schilling's testimony was not cumulative of Dr. Willinghams's testimony as to many items of projected care and their cost, as demonstrated by both her testimony and her marked-up copy of Dr. Willingham's life-care plan for Shannon. If it had been, McCoy would not have strived so hard to keep it out, and OGA and Dr. Gunn to have it admitted. Further, neither the trial court, the court of appeals, nor this Court have concluded that it was cumulative.

Which brings us to the question of whether the evidence was "so one-sided" that Dr. Schilling's testimony likely would have made "no difference in the judgment." See Cent. Expressway Sign Assocs. , 302 S.W.3d at 870 . The "so one-sided" question necessarily includes considering both Dr. Willingham's testimony and the evidence and testimony which would have been introduced through Dr. Schilling. If it were otherwise, then the testimony of almost every expert would be so one-sided that excluding opposing expert opinions would be harmless error.

Dr. Willingham testified extensively about his training, experience in treating patients, experience in teaching medical students, lectures he had given, and other matters to bolster the opinions he testified to. However, Dr. Schilling's testimony showed that she was eminently qualified to opine as to and calculate the expenses likely to be necessary for Shannon's future medical care. Dr. Schilling testified that she was the medical director of the rehabilitation department at a Houston hospital;

taught fourth-year residents in that area of medicine; was on the faculty at Baylor College of Medicine; reviewed complaints against physicians in the area of physical medicine and rehabilitation for the Texas State Board of Medicine, as well as Medicare and the Texas Workers' Compensation Commission; and performed peer reviews regarding other practitioners in the field of physical medicine and rehabilitation. But because the trial court excluded Dr. Schilling's testimony, the jury never heard about her training, extensive experience, and expertise.

Further, the jury heard Dr. Willingham testify about his personal examination of Shannon during the course of formulating his opinions. He described her conditions based on his personal observations as well as the medical records he reviewed. He described the records he reviewed in some detail and explained the amount of time he spent in reviewing them and analyzing them as part of reaching his opinions, in preparing his life-care plan for Shannon, and in estimating the costs of her future care. But the jury did not hear of Dr. Schilling's similar foundation for her testimony: her personal examination of Shannon, her detailed review of Shannon's records, the amount of time she spent in analyzing Shannon's probable future medical needs, her experience in caring for persons, such as Shannon, with severely debilitating conditions and her experience in preparing life-care plans.

So why would the jury give similar weight-or even any weight-to Dr. Schilling's views that were briefly referenced and depreciated by Dr. Willingham when Dr. Schilling's qualifications and other testimony were not admitted? The answer, logically, is that it would not. Even more, the fact that the jury knew the defendants hired Dr. Schilling to examine Shannon and review the life-care plan prepared by Dr. Willingham could well have led it to conclude that because Dr. Schilling's testimony was not presented, Dr. Willingham's testimony was not only uncontroverted, but that it was essentially uncontrovertible. Under the circumstances, it is hard to conceive of a jury giving much, if any, credence to the limited part of Dr. Schilling's testimony that it heard via the route of hearsay through Dr. Willingham.

Dr. Willingham gave his opinions as to a range of future medical care expenses, including projections of approximately $6.9 million for a home care option and $7.24 million for a facility-based care option. But through the filter of McCoy's retained expert, Dr. Willingham, the jury received a limited view of Dr. Schilling's opinions regarding Shannon's future medical care expenses. Dr. Willingham noted that Dr. Schilling projected $3.3 million as being necessary for home care and $6.7 million for facility-based care. He referenced a copy of a life-care plan he had prepared and that had been marked up by Dr. Schilling, and testified as to his disagreement with her opinions. After noting her projections for care, he opined that Dr. Schilling's view was, in effect, to divide the amount needed for Shannon's care in half. Thus, the jury heard in very cursory manner of Dr. Schilling's opinions.

The jury found $7,242,403 for future medical care-precisely the amount testified to by Dr. Willingham. That is not surprising, given that the jury heard only Dr. Willingham's testimony regarding such care. Dr. Schilling's excluded testimony, in part, would have been that approximately $3.3 million, or 46% of the amount the jury found for future care, would have been reasonable for a home care situation. Her testimony as to the probable cost of future care in a facility setting was over $500,000 less than the amount Dr. Willingham testified to. While $500,000 is less than 10% of the amount that the jury found for future care, and is much less than the $3.9 million difference between Dr. Schilling's home care projection and the jury's finding, $500,000 is nevertheless a significant amount.

The Court notes, as did the court of appeals, that the full range of future medical care expense projections were in front of the jury via the opinion testimony of Dr. Willingham. Ante at ----. But allowing a witness hired by one party to relate to the jury a filtered version of the opinion testimony of the only witness for an opposing party as to a crucial matter, then having relevant, direct testimony of the opposing party's witness as to that matter excluded and calling it a reasonably fair trial goes too far. The trial court's ruling placed the attorneys for OGA and Dr. Gunn in a no-win situation. If they did not cross examine Dr. Willingham and challenge his testimony, the jury would most likely perceive his testimony as unchallengeable. If they cross examined him extensively about Dr. Schilling's testimony, as an experienced expert witness he simply was going to repeat his opinions and disagreement with her, thus reinforcing the effect of his testimony on the jury.

If the exclusion of testimony by witnesses for opposing parties as happened here becomes the paradigm, then a minimally competent lawyer for the plaintiff-or whichever party presents evidence first-in most cases should be able to short-circuit the opposing party's presenting much evidence at all, merely by having the first, friendly witnesses testify to their version of what the opposing party and the opposing witnesses have testified to or disclosed in discovery. And it is almost guaranteed that one party's evidence presented and filtered through witnesses for the opposing party will be less adverse to the opposing party than if it were fully presented through the proponent's witnesses themselves.

In sum, I agree with the Court that the trial court erred by excluding the testimony of Dr. Schilling. But given the differences between her excluded testimony and that of Dr. Willingham regarding the key issue of the cost for future medical care for Shannon, both percentage-wise and in absolute numbers, I would hold that the excluded evidence was crucial to that key issue and the exclusion was harmful. See Diamond Offshore Servs. Ltd. , 542 S.W.3d at 552 ; see also Caffe Ribs, Inc. , 487 S.W.3d at 145 ; Cent. Expressway Sign Assocs. , 302 S.W.3d at 870 . Accordingly, I would reverse as to the future medical care damages.

II. Past Medical Expenses

The amounts recoverable in a tort action for medical care expenses must be both reasonable in price and necessary for treatment. See TEX. CIV. PRAC. & REM. CODE § 18.001(b) ; see also Haygood v. De Escabedo , 356 S.W.3d 390 , 399 (Tex. 2011). Such expenses may be proved by affidavit. Id. As relevant to the question before the Court regarding Shannon's past medical expenses, Texas Civil Practice and Remedies Code section 18.001 provides as follows:

(b) Unless a controverting affidavit is served as provided by this section, an affidavit that the amount a person charged for a service was reasonable at the time and place that the service was provided and that the service was necessary is sufficient evidence to support a finding of fact by judge or jury that the amount charged was reasonable or that the service was necessary.

(c) The affidavit must:

...

(2) be made by:

(A) the person who provided the service; or

(B) the person in charge of records showing the service provided and charge made; and

(3) include an itemized statement of the service and charge.

TEX. CIV. PRAC. & REM. CODE § 18.001(b), (c)(2)-(3).

The amounts actually paid by insurance companies, with their access to large quantities of data across many health care providers, may represent what are reasonable fees for health care services, as the Court says. Ante at 673 (citing Haygood , 356 S.W.3d at 394 (stating that agreements between willing providers and willing insurers can yield reasonable rates) ). However, whether fees paid for health care services rendered to a patient are reasonable is a much different question than the question of whether those health care services were medically necessary for the treatment of conditions caused by a particular person, event, or series of events.

OGA and Dr. Gunn objected to admission of summaries of charges and payments for Shannon's past medical care based on affidavits from representatives of three subrogation collection companies. The summaries contained information in different formats, but as relevant to the issue before us, they generally set out names of the billing providers, very brief general descriptions of services rendered by the providers, the amounts charged for the services, and the amounts paid for the services. For example, one company's summary had columns entitled "DOS Start," "DOS End," "Provider," "Service," "Diagnostic Code," "Billed," and "Paid" for each item. An example of the entries is one that listed "DOS Start" as "3/24/06"; "DOS End" as the same date of "3/24/06"; "Provider" as "The Methodist Hospital"; "Service" as "D040"; "Diagnosis Code" as "7993"; "Billed" as "71,440.50"; and "Paid" as "20,808.60". That is it-no other information, explanation, or detail as to the services provided or cost breakdown for the services. Every DOS Start and DOS End date on the summary for a particular service are the same date, just as the above example shows for $71,440.50 of hospital services billed.

OGA and Dr. Gunn point to the language of section 18.001 and argue that the subrogation agents who executed the affidavits are not within the pool of persons whose affidavits are sufficient to support admission of, and judgment based on, past medical expenses. They also argue that the documents attached to the affidavits do not meet the requirements of section 18.001. The Court disagrees. Ante at ----. I do not. I cannot agree with the Court's conclusion that by enacting section 18.001, the Legislature intended to authorize affidavits by subrogration agents located in Kentucky, Wisconsin, and Illinois to support a finding that particular treatments by health care providers in Texas were medically necessary where there is no evidence demonstrating that the affiants had any training in medicine or even had a connection with medical services, apart from their positions in companies whose business was receiving bills and information from insurance carriers and then asserting the insurers' rights of subrogation.

A. The Subrogation Agents as Affiants

McCoy used affidavits by three subrogation agents to support the claim of $703,985.98 for Shannon's past medical expenses. One affidavit was executed in Oldham County, Kentucky, by Carroll C. Rawlings. According to the affidavit, Rawlings was the Subrogation Team Manager for the Rawlings Company, whose principal place of business was on Eden Parkway in LaGrange, Kentucky. She averred that the itemized statement of claims attached to her affidavit related to Shannon's care from September 14, 2004, to December 28, 2005; the claims were paid by Aetna; and the "information contained in the records was transmitted to [her] in the regular course of business by an employee or representative of the Rawlings Company who had personal knowledge of the information." The specific language of her affidavit, in relevant part, was as follows:

Attached to this affidavit are records that provide an itemized statement of the claims paid by Aetna related to the medical care provided to Shannon McCoy from September 14, 2004 to December 28, 2005.... The attached billing records are kept by me in the regular course of business. The information contained in the records was transmitted to me in the regular course of business by an employee or representative of The Rawlings Company who had personal knowledge of the information. The records were made at or near the time or reasonably soon after the time that the service was provided.... I am familiar with the customary charges for like or similar services rendered in this vicinity and I know that the charges reflected on these records are reasonable for like or similar services rendered in this vicinity .... The services provided were necessary and the amount charged and paid for the services were reasonable at the time and place the services were provided .

(Emphasis added).

The second affidavit was executed in Waukesha County, Wisconsin, by Christi-Jo Buchanan, a Subrogation Specialist II for Meridian Resource Company, L.L.C., whose principal place of business was 20725 Watertown Road, Waukesha, Wisconsin. Buchanan's affidavit was essentially the same as Rawlings's affidavit except she averred that the claims itemized in the records attached to her affidavit covered Shannon's care from September 14, 2004, to December 23, 2007; they were paid by Unicare; and the information in the records was transmitted to her in the regular course of business by an "employee or representative of Meridian Resource Company, L.L.C. who had personal knowledge of the information."

The third affidavit was executed in Cook County, Illinois, by Patricia A. Heeney, the general counsel for Vengroff, Williams and Associates, Inc., whose principal place of business was 7101 North Cicero Avenue, Lincolnwood, Illinois. Heeney's affidavit was similar in substance to those of Rawlings and Buchanan. She averred that the claims itemized in records attached to her affidavit covered Shannon's care from March 6, 2006, to March 23, 2011; they were paid by the Federal Express Benefit Plan through its claims administrators, Cigna and Anthem Blue Cross Blue Shield; and "information contained in the records was transmitted to [her] in the regular course of business by an employee or representative of the claims administrators, Cigna Healthcare and Anthem Blue Cross Blue Shield who had personal knowledge of the information." But Heeney did not state in her affidavit that the services provided to Shannon were necessary; she stated only that "[t]he amount charged and paid for the services were reasonable at the time and place that the services were provided."

The contents of the affidavits and their attachments, simply put, do not make sense as to their purpose of proving the reasonableness and necessity of Shannon's past medical expenses, as is explained more fully below. But to briefly summarize:

1. The documents attached to the affidavits are not itemized statements as the affidavits say they are. They are one-line summaries of billings and collections.

2. The documents attached to the affidavits are not billing records as the affidavits say they are. They are summaries of billing records.

3. The affidavits of Rawlings and Buchanan circularly say that the information in the attached records was transmitted to their companies by an employee or representative of the subrogation company who had personal knowledge of the information, although the information in the attached summaries was for billings and collections as to numerous, diverse medical providers in Texas. The affidavit of Heeney was of the same nature, except in her affidavit Heeney says that an employee or representative of the claims administrators, Cigna Healthcare and Anthem Blue Cross Blue Shield had personal knowledge of the information in the attached summaries and transmitted it to Vengroff, Williams and Associates.

4. The affidavits of Rawlings and Buchanan, presented to support charges made in Houston, say that Rawlings and Buchanan, as affiants, are familiar with charges made "in this vicinity"-that is, in Kentucky and Wisconsin, respectively-and that the charges are reasonable for charges made in "this vicinity." Then, switching tracks, Rawlings and Buchanan aver that the services provided were necessary and the charges were reasonable "at the time and place the services were provided"-that is, in Houston, a completely different area of the country. Heeney's affidavit is better in that she avers that "[t]he amount charged and paid for the services were reasonable at the time and place that the services were provided." But she says nothing about whether the services were necessary.

As noted, Rawlings and Buchanan stated in their affidavits that they were familiar with customary charges for "like or similar services rendered in this vicinity " and that the charges reflected on the records attached to their affidavits "are reasonable for like or similar services rendered in this vicinity ." (Emphasis added). The vicinity of Rawlings's principal office and the location where Carroll Rawlings executed her affidavit was Oldham County, Kentucky. The vicinity of Meridian Resource's principal office was Waukesha, Wisconsin, and the vicinity of where Christi-Jo Buchanan executed her affidavit was Waukesha County, Wisconsin. But averring that they are familiar with customary charges in Kentucky and Wisconsin does not speak to their familiarity with charges for services provided in Houston. And the affidavits do not demonstrate that the affiants had any knowledge about the rendering of or the customary charges for medical care in Houston. It is in that context that both Rawlings and Buchanan averred that "the amount charged and paid for the services were reasonable at the time and place that the services were provided "-that is, Houston. Moreover, the affidavits are devoid of information to show that any of the three affiants had any training or held any position that would qualify them to give competent testimony as to the necessity of the medical procedures and services to treat Shannon, even though Rawlings and Buchanan said they were. Heeney did not bother to aver that the services for which her company was seeking to recover were necessary.

The affidavits are internally inconsistent, do not substantially comply with the Legislature's intent as to requirements for this type of affidavit, and fail in several ways to support jury findings based on basic evidentiary requirements. That is understandable because the affiants were asked to verify the medical necessity of medical services rendered in another state by persons and entities with whom they had no discernable connection, that were provided to a person whose condition they had no demonstrated knowledge of, by affidavits containing language distorted to make the affidavits appear to conform to language specified by the Texas Legislature. But as demonstrated above, the affidavits do not conform to the Legislature's requirement for proof of medical expenses by affidavit. Section 18.001 is designed to allow proof by means of an affidavit as to services and charges for the services by either the provider of the services or someone in charge of records of the provider. TEX. CIV. PRAC. & REM. CODE § 18.001. And when affidavits are made by such providers or their employees or representatives, then the problems demonstrated by these three affidavits-internal inconsistency, lack of demonstrated knowledge of the services or of how the records of the services were made, and violation of basic evidentiary rules that protect the integrity of the trial process-for the most part disappear.

The Court's interpretation fails to cabin the statute within its logical limits. "We look to and rely on the plain meaning of a statute's words as expressing legislative intent unless a different meaning is supplied, is apparent from the context, or the plain meaning of the words leads to absurd or nonsensical results." Pedernal Energy, LLC v. Bruington Eng'g, Ltd. , 536 S.W.3d 487 , 491 (Tex. 2017). And we do not read parts of a statute in isolation, but read the statute as a whole, seeking the Legislature's intent. EXLP Leasing, LLC v. Galveston Cent. Appraisal Dist. , 554 S.W.3d 572 , ----, 2018 WL 1122363 (Tex. 2018). Construing section 18.001 to permit anyone, no matter what the person's training and qualifications are, and no matter how far removed from the actual provider of the medical care and that provider's employees and representatives the person is, to support a judgment by swearing under oath to the necessity of medical expenses of that provider even though the affiant neither claimed to have nor demonstrated knowledge of the services rendered except for third-hand information of an apparent cursory nature, is an absurd and nonsensical result. Id. Further, it may well subject the statute to a constitutional challenge. See In re Murchison , 349 U.S. 133 , 136, 75 S.Ct. 623 , 99 L.Ed. 942 (1955) ("A fair trial in a fair tribunal is a basic requirement of due process."). However, as discussed below, reading sections 18.001 and 18.002 together compels the conclusion that the Legislature did not intend such results as will flow from the Court's construction of the statutory language.

B. The Billing Summaries

As for whether the affidavits comply with sections 18.001(c)(2) and 18.001(c)(3) requiring the affidavit to "include an itemized statement of the service and charge," section 18.002 demonstrates the Legislature's intent by means of providing exemplar forms for the affidavits. Id. § 18.002(c). 1 The exemplars demonstrate legislative intent that documents attached to section 18.001 affidavits must be (1) itemized as to the services provided and charges for the services and (2) the records either of (a) the person who performed the services or records based on information transmitted by either the person who performed the services or an employee or representative of the person who performed the services-provided the employee had knowledge of the services provided, or (b) persons with knowledge of the services provided, or records made based on information transmitted by persons with knowledge of the services provided. In all instances, however, the exemplars, when considered in conjunction with the language of section 18.001(c)(2), demonstrate legislative intent that to comply with the requirements of section 18.001, affidavits must be made by certain categories of persons, as discussed in the preceding section, and that documents attached to the affidavits must show that the services and charges were made by persons with knowledge of the services rendered or by persons who received information from persons with knowledge of the services rendered.

The documents attached to the affidavits in this case do not come close to measuring up. Rawlings, Buchanan, and Heeney attached to their affidavits what apparently are documents generated by each of their respective subrogation agencies, as indicated by the fact the documents attached to each of the three affidavits have different formats with different categories of information. The documents fairly clearly are not bills from medical providers, because each document lists numerous different providers. Rather, based on the information in the affidavits, they can only be summaries of information received from the payer entities. According to the affidavits, the amounts paid by the three payer entities were as follows: Aetna-$61,428.60; Unicare-$322,644.30; and Federal Express Benefit Plan-$319,912.99. But, according to the affidavits, the entities do not claim to have had any part in providing the services rendered to Shannon. The documents are not itemized billing records from the medical providers; instead, the attachments to the affidavits are cursory summaries of charges by providers and payments on those charges by the payer entities. Thus, the statements in the affidavits that "Attached to this affidavit are records that provide an itemized statement of the claims paid" are patently false. The affiants refer to the "attached billing records" as having been kept by the affiants or the subrogation entities in the regular course of business. While the documents attached to the affidavits may have been kept in the regular course of business by the subrogation entities, the subrogation documents on their face are not billing records. The affiants each circularly state that the attachments to their affidavits are based on information transmitted to the affiant by an employee or representative of the subrogation company itself, or, in the case of Heeney, by claims administrators for Federal Express Benefit Plan "who had personal knowledge of the information." But none of the affiants state that the persons who transmitted the information had knowledge of the services provided or the necessity of the services, as the section 18.002 exemplar affidavits show the Legislature intended.

I would hold that the trial court erred by overruling objections of OGA and Dr. Gunn to the admission of the documents attached to the subrogation agents' affidavits. Absent the documents, there was no evidence to support the finding as to Shannon's past medical expenses. Accordingly, I would reverse the court of appeals' judgment on this issue.

And even if the documents attached to the affidavits of Rawlings and Buchanan were properly admitted, the documents attached to the affidavit of Heeney were admitted in error. She did not state in her affidavit that the services in the documents were necessary for Shannon's treatment. Thus, there is no evidence to support that part of the judgment awarding past medical expenses paid by the Federal Express Benefit Plan.

III. Conclusion

I would reverse the judgment of the court of appeals as to both the future and past medical care expenses and remand to the trial court for further proceedings.

In the alternative, I would reverse the judgment as to the past medical expenses paid by the Federal Express Benefit Plan and render judgment that McCoy take nothing as to those expenses.

I respectfully dissent.