The defendants in an antitrust suit appeal an order partially denying their challenge to a jury verdict in favor of the plaintiff, Carlson & Erickson Builders, Inc. (C&E), a general building contractor. The jury found that Lampert Yards, Inc., a lumber supplier, and Portside Properties, Inc.,1 a contractor in competition with C&E, violated the unfair trade practice provisions of § 133.05(1) and (2), Stats.2 *225Specifically, the jury found that the defendants engaged in the secret payment of unearned discounts, special services or privileges not extended to all purchasers upon like terms and conditions.3 The jury also found that Lampert violated the provisions of § 133.04,4 by engaging in price discrimination between *226different purchasers with intent to injure or destroy competition. The jury found that Lampert and Portside did not enter into an agreement to restrain trade as prohibited by § 133.03. The jury awarded damages in the sum of $177,100. Before computing C&E's treble damages as provided in § 133.18(l)(a),5 the trial court entered a remittitur order, granting C&E the option of a reduced verdict of $67,907.65 (trebled) or a new trial on damages.

The defendants argue that the trial court erred by instructing the jury to apply the ordinary burden rather than the middle burden of proof, and that the evidence was insufficient to sustain the verdict. Port-side separately argues: (1) § 133.05(2), STATS., requires proof that it knowingly received benefits prohibited by § 133.05(1), Stats.; and (2) the statute is unconstitutionally vague. Lampert separately argues: (1) § 133.05 violates the constitutional equal protection and the commerce clause; (2) "meeting competition" and "cost justification" defenses were erroneously denied as to the claim under § 133.05; and the "functional discount" defense was erroneously denied as to the claim under § 133.04; and (3) the trial court erred by denying its motions for summary judgment, directed verdict and judgment notwithstanding the verdict.

*227We conclude that the defendants were entitled to the middle burden of proof. Further, although Portside initially waived the issue, see note 3, because the matter must be retried, C&E must prove that Portside knew that any unearned discounts, special services or privileges it received were secret and that they were not extended to all purchasers on like terms and conditions. We reject the defendants' remaining arguments. Because we remand for a new trial, we also need not address C&E's cross-appeal challenging the remittitur order. We therefore reverse and remand for a new trial on both liability and damages.6

BURDEN OF PROOF IN ANTITRUST CLAIMS

We are convinced that civil damage claims alleging violations of § 133.04 and 133.05, STATS., require the "middle" standard of proof: clear, satisfactory and convincing evidence. Over defense objection, the trial court applied the lower or "ordinary" burden.7

C&E concedes an absence of controlling Wisconsin precedent, but notes that ch. 133, Stats., is drawn *228largely from federal antitrust law, and that state courts will "look to the federal courts for guidance" where "Wisconsin case law [is] scarce." Independent Milk Producers Co-op v. Stoffel, 102 Wis. 2d 1, 6-7, 298 N.W.2d 102, 104 (Ct. App. 1980).

Wisconsin case law on the burden in civil actions involving crime-like torts is not scarce. That law consistently demonstrates a special interest in protecting defendants so accused, or those who face enhanced civil liability damages. We require the middle burden where there may be "a finding against a defendant who will be subjected to the stigma attached to the commission of certain classes of acts." Wangen v. Ford Motor Co., 97 Wis. 2d 260, 300, 294 N.W.2d 437, 458 (1980) (requiring the middle burden in punitive damage claims) (quoting Layton School of Art & Design v. WERC, 82 Wis. 2d 324, 362-63, 262 N.W.2d 218, 236 (1978)). This requirement extends to "civil actions which involved fraud, undue influence, criminal acts, reformation, mutual mistakes, and others, which public policy requires to be proved by evidence which is clear, satisfactory and convincing." Madison v. Geier, 27 Wis. 2d 687, 692, 135 N.W.2d 761, 763 (1965).8

*229In respect to the preceding standards, ch. 133, STATS., provides for treble damages and, to that extent, it is penal and not remedial in nature. Open Pantry Food Marts v. Falcone, 92 Wis. 2d 807, 811, 286 N.W.2d 149, 151 (Ct. App. 1979). The choice of a given standard of proof "serves to allocate the risk of error between the litigants and to indicate the relative importance attached to the ultimate decision." Addington v. Texas, 441 U.S. 418, 423 (1979). Unfair trade practices are punishable as criminal conduct. Section 133.04 deals with an "intent of injuring or destroying competition." Section 133.05 proscribes "an unfair trade practice" of "secret payment[s] . . . "tending to injure a competitor or... tending to destroy competition_" They subject a civil violator to a stigma no less dishonorable than other tortious conduct that must be proven to an elevated degree of certainty. These defendants must therefore be held liable only upon a finding that their actions were proven by evidence that is clear, satisfactory and convincing. While this may be inconsistent with federal law,9 to hold otherwise would render the law of Wisconsin internally inconsistent.

ELEMENT OF KNOWLEDGE IN § 133.05, Stats.

Portside contends that failure to require a buyer knowingly induce, solicit or receive secret rebates, not extended to all purchasers purchasing upon like terms and conditions, permits the imposition of treble dam*230ages upon an innocent person. C&E responds that Portside abandoned this issue by failing to raise it in the trial court. It also asserts that C&E's contention is contrary to the plain language of the statute. While we agree that a waiver occurred, the same issue will no doubt arise on remand. We therefore address it.

A statute is ambiguous when it is capable of being understood by a reasonably well-informed person in either of two or more senses. Kearney & Trecker Corp. v. DOR, 91 Wis. 2d 746, 753-54, 284 N.W.2d 61, 65 (1979). Even where a statute appears unambiguous on its face, it can be rendered ambiguous by its interaction and its relation to other statutes. State v. White, 97 Wis. 2d 193, 198, 295 N.W.2d 346, 348 (1980). We conclude that § 133.05(2), STATS., is ambiguous. Subsection (3) of the statute states that any person "knowingly" violating its terms may be criminally prosecuted, while subsec. (2) contains no reference to knowledge or state of mind. Arguably this denotes, as C&E contends, that the legislature contemplated the requirement of knowledge only in criminal prosecutions.

The general legislative intent expressed in § 133.01, Stats., however, suggests otherwise. That intent is to prohibit unfair business practices.10 A *231buyer who receives prohibited benefits, innocent of any knowledge that the seller is so acting, is not engaged in an unfair trade practice. Further, for much the same reason that the penal damages of § 133.18 require a greater than ordinary burden of proof, we conclude that a buyer must know that unearned discounts, special services or privileges are secret and not extended to all purchasers purchasing on like terms and conditions. Finally, the nature of the practices prohibited by § 133.05, suggest that proof of the buyer's knowledge is no great barrier to a civil claim. Ordinarily, in the process of proving secret activities, the same evidence would circumstantially demonstrate the necessary knowledge.11

SUFFICIENCY OF THE EVIDENCE

The defendants argue that the evidence was insufficient to support the verdict. Because we remand for a new trial, we will not address the evidence in detail. The trial court instructed the jury that the elements of *232price discrimination require that (1) the commodities involved in the sales being compared were of like grade and quality; (2) the prices charged were discriminatory, that is, different; (3) the price difference was for the purpose or with the intent of injuring competition or a competitor; and (4) the discriminatory pricing caused the plaintiff injury. The court instructed that the elements of proof of a secret discount are: (1) the payment or allowance of unearned discounts, special services or privileges; (2) made in secret; and (3) that these acts injure or tend to injure a competitor or to destroy competition.

The transcript of the nine-day jury trial reveals extensive evidence on each of the preceding elements. Further, based upon our examination of the record, the trial court properly denied defense motions for summary judgment, directed verdict and judgment notwithstanding the verdict.

DEFENSES AVAILABLE TO LAMPERT

The trial court instructed the jury on a "meeting the competition" defense and a "cost justification" defense relating to the § 133.05, Stats., claim, and a "functional discount" defense as to the § 133.04 claim. Lampert argues that each of these defenses is equally applicable to both statutes. C&E responds that even if this were true, any error would be harmless because the jury rejected the defenses in relation to the claims to which they were given. Because Lampert's argument is unsupported by authority and otherwise inadequately briefed, we decline to review it. See In re Estate of Balkus v. Security First Nat'l Bank, 128 Wis. 2d 246, 255 n.5, 381 N.W.2d 593, 598 n.5 (Ct. App. 1985).

*233VOID FOR VAGUENESS AND OTHER CONSTITUTIONAL ARGUMENTS

Portside argues that § 133.05, Stats., is void because "unearned discounts" and "like terms and conditions" are not defined. A statute is presumptively constitutional, and a party challenging the act has the burden of establishing unconstitutionality beyond a reasonable doubt. Madison v. Hyland, Hall & Co., 73 Wis. 2d 364, 385, 243 N.W.2d 422, 433 (1976). The void for vagueness concept rests upon the constitutional principle that procedural due process requires fair notice and proper standards for adjudication. State ex rel. Hennekens v. River Falls Police & Fire Comm'n, 124 Wis. 2d 413, 420, 369 N.W.2d 670, 674 (1985). If the statute is so obscure that persons of common intelligence must necessarily guess as to its meaning and differ as to its applicability, it is unconstitutional. State v. Zwicker, 41 Wis. 2d 497, 507, 164 N.W.2d 512, 517 (1969). Where a great and varied number of offenses may come within a statutory category, they defy a precise statutory definition. See id. Impossible standards of specificity are not required. Id. (citing Jordan v. De George, 341 U.S. 223, 231 (1951)). Moreover, the trial court gave the defendants' requested instruction as follows:

Defendants have raised the functional discount defense to the unearned discount claim. "Functional discounts" are given by a seller to a buyer based on the buyer's performance of marketing functions for the seller's product. In this case, Portside or DCMS claim that they received the discounts because they performed certain services, including design functions, drafting of blueprints, preparation of material takeoff lists, materials cost estimates, *234coordinating of deliveries of building materials to construction sites, and also that they assumed the risks of error in estimating the materials necessary for construction projects, which other contractors, including the plaintiff, Carlson & Erickson, ordinarily relied on Lampert to perform.

The jury was informed that if the lower price granted to Portside was justified as a functional discount, then the jury must find for the defendants on the unearned discount claim. We conclude that Portside fails to demonstrate that the statute denies due process and is thereby void for vagueness.

Lampert raises constitutional challenges relating to the commerce clause and equal protection. Its arguments are unpersuasive, and we reject them. The remittitur order is reversed and the matter remanded for a new trial consistent with this opinion.

By the Court. — Order affirmed in part; reversed in part and cause remanded with directions. No costs on appeal.

{concurring). I concur only for the purpose of urging that the courts reconsider having different burdens of proof in civil cases. As a former trial judge, I presided over hundreds of civil jury trials, and I seriously doubt that any juror can realistically tell the difference between the "ordinary" and "middle" burden of proof. Under the "ordinary" burden of proof, the jury is told: "The burden of proof... is to satisfy you to a reasonable certainty by the greater weight of the credible evidence ...." Wis JI — CIVIL 200. In comparison, under the "middle" burden of proof, the jury is told: "The burden of proof ... is ... to convince you to a *235reasonable certainty by evidence that is clear, satisfactory, and convincing...WlS JI — CIVIL 205. Arguably, one could contend the "ordinary" burden of proof is the higher standard.

As lawyers and judges, we somehow perceive a difference between the burden of proofs. Realistically, I say there is no difference. The important differences are the elements to be proven for liability, not the burden of proof. For those who face an enhanced civil liability or who will be subjected to a stigma if found liable, we require additional elements to be proven, such as knowledge, bad faith, conspiracy, intentional acts or misrepresentations. That should be sufficient to distinguish the various forms of civil liabilities. We should not continue to engage in this subtle difference, if any, in the burden of proofs. The courts realized this when abandoning the concept of ordinary versus gross negligence, and we should likewise abandon the concept of ordinary versus middle burden of proof.

I am authorized to state that Judge Ralph Adam Fine joins in this concurrence.