The error of equal error rates

Abstract

In most civil litigation, plaintiffs are required to prove their cases by a bare ‘preponderance of the evidence’. This usually is taken to mean that the probability that their version of the disputed events is true exceeds one-half. But why use this particular figure? Why not some smaller number, like one-third, or some larger number, like three-quarters? One common answer is that the p > 1/2 rule minimizes the expected ‘losses’ (Kaye, 1999), ‘disutilities’ (Kaplan, 1968), ‘regret’ (Lempert, 1977), ‘costs’ (Posner, 1999), or ‘errors’ (Kaye, 1980a) in verdicts that turn on these events. This analysis has been called the Bayesian Decision Theory justification or explanation of the p > 1/2 rule (Kaye, 1988), and it has been presented as the ‘reigning theory’ of the law’s burdens of persuasion (Kaye, 1987a). But not all legal analysts accept this theory. Several distinguished scholars have tried to defend the p > 1/2 rule on other grounds. They see it as advancing a different objective—allocating erroneous verdicts across plaintiffs and defendants in equal numbers. In Evidence: Text, Cases, and Problems, a rich and thoughtful textbook on the law of evidence, Professors Ronald J. Allen, Richard B. Kuhns, and Eleanor Swift (Allen et al., 1997, p. 828) identify what they call ‘the premises underlying the preponderance rule’ as follows: The preponderance rule incorporates an underlying assumption concerning the participants in litigation: that plaintiffs as a class and defendants as a class generally ought to be treated equivalently. The reason for this assumption is that before a case is resolved, one cannot know who should win; it is as likely that the defendant should win as the plaintiff ... . Without knowing the facts, it seems just as likely that the defendant is refusing to pay what is owed as that the plaintiff is attempting to obtain an undeserved benefit.