David Carpenter (employee), a custodian at an elementary school in the city of Woburn (employer), died at the age of fifty-two while at work and after operating a heavy, commercial snow blower. He experienced “sudden cardiac death” (SCD) due to ventricular arrhythmia.1 The employer’s workers’ compensation insurer, MEA Workers’ Compensation SIG (insurer), appeals from a decision of the reviewing board of the Department of Industrial Accidents (board) affirming a decision of an administrative judge awarding burial expenses under G. L. c. 152, § 33, and survivor benefits under G. L. c. 152, § 31, to the employee’s widow. The insurer argues that the board incorrectly applied G. L. c. 152, § 7A (§ 7A),2 to the facts of this case; the board erroneously declined to consider the insurer’s defense *438pursuant to G. L. c. 152, § 1 (7A) (§ 1 [7A]),3 concerning the effect of the employee’s preexisting coronary artery disease (CAD) on the element of causation; and the administrative judge erroneously applied § 1 (7A). We transferred the case from the Appeals Court, see G. L. c. 152, § 12 (2), on our own motion. We reject these arguments and affirm the board’s decision, as modified herein.

1. Background. We summarize the facts found by the administrative judge and adopted by the board.4 On Monday, January 24, 2005, the employee was working with a coworker clearing a large amount of heavy snow from school grounds. It had snowed over the course of the weekend, leaving an accumulation of two feet of snow, and as a result, the school was closed. After arriving at work at about 7:00 a.m., the employee spent the morning operating a large, commercial-model snow blower, shoveling a tall snow drift with his coworker, and replacing a broken cotter pin on the snow blower. Although the snow blower was self-propelling, it weighed about 200 pounds and had to be manually maneuvered from side to side and around sharp corners.

At 12:15 p.m., the employee went to lunch with his wife. During lunch, he did not voice any complaints or indicate that he was not feeling well. He returned to work at approximately 1:30 p.m.. At the time, it was cold outside, with temperatures ranging between nineteen and thirty-three degrees Fahrenheit.

The employee resumed operating the snow blower to improve the paths he had completed before lunch. His coworker heard the motor of the snow blower running continuously and, at approximately 1:50 p.m., went to check on the employee. He found the employee motionless, leaning against a snow drift “higher than a man” beside the snow blower. Emergency medical person*439nel were summonsed and determined on arrival that the employee had no pulse. By ambulance, the employee was transported to a nearby hospital, where he was pronounced dead. The employee had experienced SCD.

2. Standard of review. An aggrieved party may seek judicial review of a decision of the board concerning workers’ compensation benefits. G. L. c. 152, § 12 (2). As specified in G. L. c. 152, § 12 (2), we review the board’s decision in accordance with the standards set forth in G. L. c. 30A, § 14 (7) (a)-(d), (f), and (g). Scheffer’s Case, 419 Mass. 251, 257-258 (1994). Pursuant to these statutory standards, we may reverse or modify the board’s decision when it is “[i]n violation of constitutional provisions,” “[i]n excess of the statutory authority or jurisdiction of the agency,” “[bjased upon an error of law,” “[mjade upon unlawful procedure,” “[ujnwarranted by facts found . . . where the court is constitutionally required to make independent findings of fact,” or is “[ajrbitrary or capricious, an abuse of discretion, or otherwise not in accordance with law.” G. L. c. 30A, § 14 (7) (a)-(d), (f), and (g). Under these standards, in determining whether the board properly affirmed the administrative judge’s findings, we consider “whether the decision is factually warranted and not ‘[ajrbitrary or capricious,’ in the sense of having adequate evidentiary and factual support and disclosing reasoned decision making.” Scheffer’s Case, supra at 258. See Robinson’s Case, 416 Mass. 454, 457 (1993) (standards identified in G. L. c. 152, § 12 [2], “may overlap with the substantial evidence standard”). Where a statute is involved, “[ajlthough ‘[tjhe interpretation of a statute by the agency charged with primary responsibility for administering it is entitled to substantial deference,’ Gateley’s Case, 415 Mass. 397, 399 (1993), ultimately the ‘duty of statutory interpretation is for the courts.’ Slater’s Case, 55 Mass. App. Ct. 326, 330 (2002).” Moss’s Case, 451 Mass. 704, 709 (2008).

3. Discussion, a. Application of § 7A. The insurer asserts that the employee’s SCD was inevitable and its occurrence at work was coincidental. Consequently, the insurer states that the board erred in affirming the administrative judge’s decision, arguing that he improperly applied § 7A on the issue of the causal link between employment and death. In support of this argument, the insurer contends that the administrative judge’s decision *440reflects a misunderstanding of the medical evidence. We reject these arguments.

In Moss’s Case, supra at 707-708, we provided a comprehensive overview of § 7A, see note 2, supra:

“Section 7A is designed to ‘[t]o aid the victimf] of an unwitnessed accident’ who is unable to testify and therefore has difficulty meeting the burden of proving entitlement to compensation under G. L. c. 152. L.Y. Nason, C.W. Koziol, & R.A. Wall, Workers’ Compensation § 17.4 (3d ed. 2003). See id. at § 10.11. The section by its terms establishes prima facie evidence of four separate aspects of a workers’ compensation claim: (1) ‘that the employee was performing his regular duties on the day of injury or death’; (2) ‘that the claim comes within the provisions of [c. 152]’; (3) ‘that sufficient notice of the injury has been given’; and (4) ‘that the injury or death was not occasioned by the willful intention of the employee to injure or kill himself or another.’ G. L. c. 152, § 7A. In Anderson’s Case, [373 Mass. 813 (1977)], this court held that particularly the second of these, ‘the claim comes within the provisions of [c. 152],’ reflected a legislative intent that where an employee was found dead at his job, this fact should establish prima facie evidence that the employee’s death was causally related to his employment. The court stated:

‘For a claim to be compensable it must arise out of and in the course of employment. Clearly a causal relationship is required between the employment duties and the injury or death. In a case such as this one, where the employee was found dead at his place of employment, we construe the statute, § 7A, as establishing, inter alla, prima facie evidence of causal relationship between the employment and the injury or fatality. We believe that was the meaning intended by the Legislature, particularly in its use of the words, “and that the claim comes within the provisions of this chapter.” Anderson’s Case, [supra] at 816-817.’ ”5

An insurer may overcome the prima facie evidentiary effect *441of § 7A by assuming the burden of production. In Anderson’s Case, supra at 817, we explained:

“Prima facie evidence, in the absence of contradictory evidence, requires a finding that the evidence is true; the prima facie evidence may be met and overcome by evidence sufficient to warrant a contrary conclusion; even in the presence of contradictory evidence, however, the prima facie evidence is sufficient to sustain the proposition to which it is applicable.”

If an insurer produces “evidence sufficient to warrant a contrary conclusion,” id., on a fact set out in § 7A, a question of fact arises for an administrative judge to decide, and the judge must weigh the evidence and make credibility determinations in reaching a conclusion. Such credibility determinations are within the sole province of an administrative judge and are to be considered final by both the reviewing board and an appellate court. Lettich’s Case, 403 Mass. 389, 394 (1988). See Pilon’s Case, 69 Mass. App. Ct. 167, 169 (2007) (“Findings of fact, assessments of credibility, and determinations of the weight to be given the evidence are the exclusive function of the administrative judge”). Where, as here, “causal relation is a matter beyond the common knowledge and experience of the ordinary layman, the proof must rest upon expert medical testimony.” Buck’s Case, 342 Mass. 766, 769 (1961). “The probative value of this testimony is to be weighed by the fact-finding tribunal.” Fitzgibbons’s Case, 374 Mass. 633, 635 (1978).

Before addressing the insurer’s arguments, we summarize the findings and conclusions of the administrative judge, as well as the conclusions drawn by the board. The administrative judge first determined that the employee was in the course of his employment when he suffered SCO and that, consequently, the prima facie evidentiary effect of § 7A applied. He credited the findings of the limited autopsy conducted of the employee’s heart. The autopsy indicated that the employee was suffering from an occlusion (narrowing) of over ninety per cent of the left anterior descending coronary artery. It also revealed that the *442employee had “patchy subendocardial fibrosis [scarring of the heart] but no evidence of substantial old myocardial infarction [heart attack] and no evidence of acute injury.”

The administrative judge adopted the conclusion, reached both by one of the employee’s medical experts, Dr. Richard D. Patten, and by the insurer’s medical expert, Dr. Milo F. Pulde, that the employee’s SCD resulted from ventricular arrhythmia, see note 1, supra. As to the insurer’s defense under § 1 (7A) concerning the effect of the employee’s preexisting coronary artery disease, the administrative judge adopted Dr. Pulde’s (insurer’s expert) opinion that the employee did, in fact, suffer from a preexisting condition. Specifically, Dr. Pulde gave the opinion that, prior to his death, the employee suffered “with severe multivessel coronary disease with critical ninety-five per cent obstruction of the major left anterior descending artery . . . and less critical disease of the remaining arteries.” Dr. Pulde’s opinion was based on the records of the employee’s primary care physician (PCP), Dr. Arthur S. Kress, which indicated that the employee suffered, on January 4, 2005, from unstable angina6; and the autopsy finding of fibrosis, which, to Dr. Pulde, indicated that the employee had a severely diseased “coronary anatomy prior to [the date of death] that was causing damage to the heart muscle and that damage was identified at pathology as fibrosis or scarring.” The administrative judge credited Dr. Pulde’s opinion that the employee’s preexisting conditions “were medically consistent with severe non-work related multivessel coronary disease and unstable angina ‘that culminated in either ischemia-related [loss of sufficient blood flow to the heart] polymorphic ventricular tachycardia or substrate (subendeocardial fibrosis) [diseased, scarred heart muscle] related monomorphic ventricular tachycardia *443. . . that degenerated [on January 24, 2005,] into ventricular fibrillation and resulted in his biological death.’ ”7

The administrative judge expressly credited the following testimony of Dr. Pulde, elicited during cross-examination, concerning the effect of physical exertion on the employee’s biological death: “[A]ny type of exertion, whether one postulates that [the employee’s death was] ischemia related or substrate related . . . would have precipitated [the employee’s] sudden death and ... his sudden death was likely to occur in any context [in] any type of physical activity and would have occurred independent of that work activity on January 24, 2005” (emphasis added). The administrative judge, however, expressly rejected Dr. Pulde’s opinion that the employee’s work activities on January 24, 2005, did not trigger the medical events that culminated in his SCD.8 Instead, the administrative judge credited and adopted the opinion of Dr. Kress, stating: “[Dr. Kress], while recognizing that his patient suffered from several cardiac risk factors along with pre-existing cardiac conditions, offered his ‘medical opinion that the work that [the employee] was doing on the day in question was a major contributing factor in his death.’ ”9,10

The administrative judge concluded that “the insurer has not *444brought forward convincing evidence sufficient to overcome the prima facie presumptions afforded by § 7A.” He found that the employee suffered SCD arising out of and while in the course of his employment, and that the employee’s work activities on January 24, 2005, were “a major contributing cause of his sudden cardiac arrest and resulting death.” The administrative judge further concluded that the insurer’s defense under § 1 (7A) lacked merit.

The board concluded that the administrative judge “properly determined that § 7A applied to this case.” Concerning the administrative judge’s conclusion that Dr. Pulde’s opinion did not overcome the prima facie evidentiary effect of § 7A, the board determined that this conclusion was supported by the record and was “consistent with § 7A’s application.” Additionally, the board decided that the administrative judge’s findings provided the necessary factual predicate to support his adoption of Dr. Kress’s opinion.

Here, the insurer essentially asserts that once the administrative judge adopted some of Dr. Pulde’s opinions, it was inconsistent and erroneous not to apply them all, specifically, Dr. Pulde’s opinion that the employee’s work on January 24, 2005, had no causal relation to his death. Although the insurer did produce evidence, through Dr. Pulde’s testimony, in an attempt to overcome the prima facie evidentiary effect of § 7A (and offered Dr. Pulde’s opinion in support of its defense under § 1 [7A]), the administrative judge was not required to credit all of Dr. Pulde’s testimony, particularly when, as relevant to the issue whether the employee’s death was work-related, there existed contrary evidence, namely Dr. Kress’s opinion, which the administrative judge was entitled to credit.11 See Amon’s Case, 315 Mass. 210, 215 (1943).

*445The insurer contends that the administrative judge’s conclusion cannot stand because Dr. Kress’s opinion was predicated on two facts that were not specifically found by the judge, namely, heavy exertion and an extremely cold environment. We point out, however, that as to the latter subject, the judge found that the temperature at the time of the employee’s death was cold and ranged between nineteen and thirty-three degrees Fahrenheit. Further, there was no need for the administrative judge to make specific findings concerning the exact level of physical exertion undertaken by the employee before his death in view of the judge’s acceptance of Dr. Pulde’s opinion that any level of exertion (and not necessarily heavy or significant physical exertion as opined by Drs. Kress and Patten, see note 10, supra) could have precipitated SCD in the employee on account of the severity of his preexisting CAD (in the absence of identifying and treating the disease). It was enough that this employee, in his condition, had engaged in any degree of physical exertion at work, and this conclusion was supported by the administrative judge’s finding that, after lunch and within the hour preceding his death, the employee had cleared “an additional two to four feet of pathway” with the snow blower. The result reached by the administrative judge is consistent with, and supported by, the medical evidence in the record, and reflects a correct application of § 7A.

b. Application of defense under § 1 (7A). Although the administrative judge considered (and rejected) the insurer’s defense under § 1 (7A), the board concluded that, in cases where an employee is found dead at the workplace, only § 7A applies and the “combination injury” defense, see note 3, supra, under *446§ 1 (7A) has no application. The board reasoned that § 7A and the defense under § 1 (7A) are mutually exclusive. The board based its conclusion on the absence of language in § 1 (7A) regarding workplace deaths. In addition, the board found the combination injury defense inapplicable because, in the case of a death, “no period of disability or a need for medical treatment occurs,” as contemplated by the language of § 1 (7A). Both the employee and the insurer maintain that the defense under § 1 (7A) applies in cases of death occurring at the workplace. The issue is one of statutory construction.

“[A] statute must be interpreted according to the intent of the Legislature ascertained from all its words construed by the ordinary and approved usage of the language, considered in connection with the cause of its enactment, the mischief or imperfection to be remedied and the main object to be accomplished, to the end that the purpose of its framers may be effectuated.” Board of Educ. v. Assessor of Worcester, 368 Mass. 511, 513 (1975), quoting Industrial Fin. Corp. v. State Tax Comm’n, 367 Mass. 360, 364 (1975). “[T]he statutory language itself is the principal source of insight into the legislative purpose.” Hoffman v. Howmedica, Inc., 373 Mass. 32, 37 (1977). We “must ascertain the intent of a statute from all its parts and from the subject matter to which it relates, and must interpret the statute so as to render the legislation effective, consonant with sound reason and common sense.” Harvard Crimson, Inc. v. President & Fellows of Harvard College, 445 Mass. 745, 749 (2006).

The board’s analysis overlooks the workers’ compensation statute (Act) as a whole, as well as the circumstances pursuant to which the defense under § 1 (7A) was enacted. Significantly, the Act contemplates compensation for both work-related deaths and personal injuries; it is not solely concerned with the latter. That a death is not specifically included in the definition of the term “personal injury” is inconsequential. An examination of the definition reveals that the term does not state what a “personal injury” actually is; rather, it includes and excludes only certain categories of personal injuries, and notably neither includes nor excludes a death.12

*447“The various sentences of § 1 (7A) were adopted piecemeal over time ... as a series of legislative responses to specific court decisions and perceived needs for targeted reform.” Cornetta’s Case, 68 Mass. App. Ct. 107, 114 (2007). In 1991, the Legislature inserted the third and fourth sentences of the definition of personal injury, setting forth the combination injury defense into the fourth sentence of the definition. See St. 1991, c. 398, § 14. The 1991 amendments to the Act “came in response to declining economic conditions and rising costs of workers’ compensation insurance.” Taylor’s Case, 44 Mass. App. Ct. 495, 500-501 (1998).

Application of the combination injury defense pursuant to § 1 (7A) effectuates the Legislature’s targeted reform by limiting compensation in certain cases where a noncompensable preexisting condition exists. The obvious concern was to withhold compensation in circumstances where a noncompensable, preexisting condition merely happens to manifest itself at the employee’s workplace and has no relation to the employee’s actual work. See Maggelet’s Case, 228 Mass. 57, 61 (1917). This concern exists not only in circumstances where the noncompensable preexisting condition combines with a compensable injury or disease to result in personal injury, but also in cases where the combination injury results in death.

The insurer asserts that death cannot constitute an injury under the combination injury defense of § 1 (7A) and that, therefore, the employee’s SCD can never be a compensable *448injury. This interpretation would render § 7A superfluous. Further, the compensable injury is not the employee’s death but rather, in this case, the ventricular arrhythmia or fibrillation (that resulted from either the polymorphic or monomorphic ventricular tachycardia). The employee’s death was the ensuing disability. The expert medical evidence underscores this distinction. Dr. Patten explained (and his testimony in this regard was not contradicted) that a person experiencing ventricular fibrillation may, within a certain amount of time (measured in minutes), be “revived” or be “convert[ed] . . . back to a normal rhythm.”13 Thus, with these particular types of injuries, death may occur, but is by no means always certain, and as such, the occasion of death cannot be considered the injury under § 1 (7A).

Consideration of the effect of a preexisting condition in situations involving a work-related death is not inconsistent with § 7A. If a noncompensable preexisting condition does not exist, only § 7A will apply (if the employee’s death occurred at the workplace), and the defense under § 1 (7A) will not be implicated.

When there is a noncompensable preexisting condition that combines with a compensable injury and results in a workplace death, application of the defense somewhat alters the insurer’s burden of production in rebutting the prima facie evidentiary effect of § 7A. First, the insurer must present evidence that a noncompensable preexisting condition existed at the time of the employee’s death. The production of this evidence, together with appropriate medical expert evidence on the issue of causation, would allow the insurer to argue that the employee’s death was caused solely by the preexisting condition. Second, where an employee’s workplace death does follow a combination injury (and the insurer does not argue that the death resulted solely from the noncompensable preexisting condition), the insurer will need to submit appropriate medical expert evidence that the compensable part of the combination injury (here, the ventricular arrhythmia or fibrillation) was not a major cause of the death, which includes a causal showing that the compensable *449part of the combination injury was not brought on by the employee’s work activities.14 The employee, with whom the ultimate burden of production and persuasion lies, may respond by proffering appropriate expert medical evidence to contradict the evidence of the insurer. See Viveiros’s Case, 53 Mass. App. Ct. 296, 299 (2001). For these reasons, the defense under § 1 (7A) and the prima facie evidentiary effect of § 7A are not mutually exclusive.

c. Administrative judge’s determinations under § 1 (7A). We reject the insurer’s contention that the administrative judge’s decision does not sufficiently analyze the element of causation under § 1 (7A). The judge’s extensive factual findings, supplemented with uncontested evidence from the record, provide the necessary factual predicate to support his conclusions, including his rejection of the insurer’s defense under § 1 (7A).

The board incorrectly concluded that the defense under § 1 (7A) and the prima facie evidentiary effect of § 7A are mutually exclusive. Apart from making this erroneous conclusion, the board properly affirmed the administrative judge’s decision. We therefore affirm the decision of the reviewing board, as so modified.

So ordered.