Error and exaggeration in the presentation of DNA evidence at trial

Abstract

This Article identifies some of the subtle, but common, exaggerations that have occurred at trial, and classifies each in relation to the three questions that are suggested by the chain of reasoning sketched above: (1) Is a reported match a true match? (2) Is the suspect the of the trace? (3) Is the suspect the perpetrator of the crime? Part I addresses the first question and discusses ways of defining and estimating the false positive rates at DNA laboratories. Parts II and III address the second and third questions, respectively. These sections introduce the source probability error and ultimate issue error and show how experts often commit these errors at trial with assistance from attorneys on both sides. Part IV introduces two related exaggerations, the P(Another Match) error and the numerical conversion error. Part V provides a simple and general explanation for the persistence the errors identified. Part VI concludes with a discussion of the ways in which scientists can take advantage of their roles as teachers, expert witnesses, and researchers to educate the courts about the meaning and limits of probabilistic DNA evidence.

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