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67 U. Colo. L. Rev. 859 (1996)
On Conveying the Probative Value of DNA Evidence: Frequencies, Likelihood Ratios, and Error Rates

handle is hein.journals/ucollr67 and id is 883 raw text is: ON CONVEYING THE PROBATIVE VALUE OF DNA EVIDENCE: FREQUENCIES, LIKELIHOOD RATIOS, AND ERROR RATES JONATHAN J. KOEHLER* INTRODUCTION The first numerical probability that was offered in the 0. J. Simpson case came from the defendant himself when he pleaded Absolutely, 100% not guilty.1 There isn't much question about what Mr. Simpson meant by that probability statement. But hundreds of other probability statements were offered in his trial that left considerable room for ambiguity, interpretation, and misinterpretation. Most of these probability statements were associated with DNA evidence that purportedly linked Mr. Simpson to the crime. What do the numbers mean? How should such evidence be presented to jurors? What exactly is meant by the evidence? What role, if any, should the possibility of error play in the interpretation of DNA frequencies? Part I of this paper discusses the issue of what the frequen- cies associated with DNA evidence do and do not mean. Part II describes an alternate way of presenting DNA statistics in court based on Bayesian likelihood ratios. This part also addresses issues associated with identifying the hypothesis of interest and characterizing the evidence. Part III offers a likelihood ratio and estimates its magnitude based on proficiency test data that were analyzed by Koehler, Chia, and Lindsey.2 Part IV identifies and rebuts arguments that have been offered for ignoring the possibility of error when computing DNA likelihood ratios. Part V considers whether likelihood ratios belong in court and concludes that they may not if experts and jurors do not under- stand them. Part VI reports an empirical study that compares * Associate Professor of Behavioral Decision Making, Graduate School of Business and Law School, University of Texas at Austin. Ph.D., 1989, University of Chicago. 1. B. Drummond Ayers Jr., AbsolutelyNot Guilty, A Confident Simpson Says, N.Y. TIMES, July 23, 1994, at A6. 2. Jonathan J. Koehler et al., The Random Match Probability (RMP) in DNA Evidence: Irrelevant and Prejudicial?, 35 JURIMETRICS J. 201 (1995).