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Law, Probability and Risk Advance Access originally published online on July 28, 2009
Law, Probability and Risk 2009 8(3):257-276; doi:10.1093/lpr/mgp013
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© The Author [2009]. Published by Oxford University Press. All rights reserved.

Painting the target around the matching profile: the Texas sharpshooter fallacy in forensic DNA interpretation{dagger}

William C. Thompson*

Department of Criminology, Law and Society, University of California, Irvine, CA 92697, USA

* Email: william.thompson{at}uci.edu

Received on 17 November 2008. Revised on 19 April 2009. Accepted on 24 April 2009.


  Abstract

Forensic DNA analysts tend to underestimate the frequency of matching profiles (and overestimate likelihood ratios) by shifting the purported criteria for a ‘match’ or ‘inclusion’ after the profile of a suspect becomes known—a process analogous to the well-known Texas sharpshooter fallacy. Using examples from casework, informal and naturalistic experiments, and analysts’ own testimony, this article demonstrates how post hoc target shifting occurs and how it can distort the frequency and likelihood ratio statistics used to characterize DNA matches, making matches appear more probative than they actually are. It concludes by calling for broader adoption of more rigorous analytical procedures, such as sequential unmasking, that can reduce the sharpshooter fallacy by fixing the target before the shots are taken.

Keywords: DNA evidence; frequency; likelihood ratio; fallacy; sequential unmasking; DNA profile; bias; error; statistics; ACE-V

{dagger} Presented as part of the Seventh International Conference on Forensic Inference and Statistics, The University of Lausanne, Switzerland, 21st to 23rd August, 2008.


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