The prosecutor’s fallacy.

This is the name given to statistical errors that can arise when deciding the probability that a DNA sample is that of the accused.  This is potentially more meaningful than usual to Troy Brown who was convicted of sexual assault and attempted murder.  He has a twin brother.

This transposition of the conditional probability can produce results that range from the approximately correct to the grossly inaccurate. Without discussing the extent of the mathematical error, Mueller’s letter stated that this transposition was "so common it has been given a special name, the prosecutor’s fallacy."

Indeed, the fallacy abounds in the statements of judges, defense counsel, and journalists. Statistics textbooks, evidence casebooks and treatises, and judicial opinions all caution against it.

Professor Kaye has written on this issue because of a pending Supreme Court case.

In McDaniel v. Brown, (thank you SCOTUSWiki) the Supreme Court will review the use of DNA evidence in a 1994 trial for sexual assault and attempted murder. The Court granted certiorari to consider two procedural issues—the standard of federal postconviction review of a state jury verdict for sufficiency of the evidence, and the district court’s decision to allow the prisoner to supplement the record of trials, appeals, and state postconviction proceedings with a geneticist’s letter twelve years after the trial. The letter from Laurence Mueller, a professor at the University of California at Irvine, identified two obvious mistakes in the state’s expert testimony.

This essay clarifies the nature and extent of the errors in this evidence in Brown.

"False But Highly Persuasive": How Wrong Were the Probability Estimates in McDaniel v. Brown?

David H. Kaye, Distinguished Professor and Weiss Family Scholar, Pennsylvania State University, Dickinson School of Law.

Suggested citation: David H. Kaye, Commentary, "False, But Highly Persuasive": How Wrong Were the Probability Estimates in McDaniel v. Brown?, 108 Mich. L. Rev. First Impressions 1 (2009), http://www.michiganlawreview.org/firstimpressions/vol108/kaye.pdf.

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