Frontline's Expose of DNA Testing: Yes and No

The content of a recent Frontline story on “The Surprisingly Imperfect Science of DNA Testing: How a Proven Tool May Be Anything But” will come as no surprise to anyone familiar with the professional and academic literature on forensic DNA identification. The work is important, though, because the main points of the story need to be widely understood. I highlight them below. At the same time, the story relies on some putative problems that are more perceived than real. 

The story boils down to this (with annotations on the right)
Complex DNA mixtures can be tough to interpret, especially when the amounts of DNA are so small that stochastic effects in amplifying DNA sequences are important. In these situations, analysts using a variety of cues and procedures--and even following the same general procedure--can reach different conclusions. False arrests and convictions can follow.	True. See, e.g., Erin Murphy, The Art in the Science of DNA: A Layperson's Guide to the Subjectivity Inherent in Forensic DNA Typing, 58 Emory L.J. 489 (2008).
There can be a big difference between the answer to the question “What is the probability of a match between a specific DNA profile and the profile an individual picked at random?” and “What is the probability of a match between a specific DNA profile and at least one profile in a large database of profiles picked at random?”	Yes, different questions give different answers. The size of the database affects the answer to the second question, but not the first. But which question should be addressed in court? Probably neither. See, e.g., Ian Ayres & Barry Nalebuff, The Rule of Probabilities: A Practical Approach for Applying Bayes’ Rule to the Analysis of DNA Evidence, 67 Stan. L. Rev. 1447 (2015); David J. Balding, The DNA Database Search Controversy, 58 Biometrics 241 (2002); David H. Kaye, Rounding Up the Usual Suspects: A Legal and Logical Analysis of DNA Database Trawls, 87 N. Car. L. Rev. 425 (2009).
Thinking about how traces of DNA ended up where they did—and not just whose DNA ended up there—can be crucial to an investigation and prosecution. If investigators, lawyers, and jurors do not understand this, there can be mistaken arrests, prosecutions, and convictions.	A very important point. See, e.g., Peter Gill, Misleading DNA Evidence (2014); David H. Kaye, David Bernstein & Jennifer Mnookin, The New Wigmore on Evidence: Expert Evidence (2d ed. 2011).
A few things will be surprising
The 2011 Hampikian-Dror “experiment” with a complex DNA mixture was good proof that examiner bias (from knowing what detectives believed) affected two examiner's interpretations.	From the viewpoint of experimental design, a potentially important confounding variable was obviously present. See D.H. Kaye, The Design of “The First Experimental Study Exploring DNA Interpretation”, 52 Science & Justice 256 (2012). As Dror later wrote, the study was  “suggesting that the extraneous context of the criminal case may have influenced the interpretation of the DNA evidence” (emphasis added by Dror in a reply letter). This conclusion is reminiscent of a microscopic hair association reported as “suggesting that the [defendant] may have [left the hair at the crime scene].” How helpful is that?
"It’s not clear how often coincidental matches occur." Indeed, it might not be a rare event at all, considering that "a rogue Arizona state employee had run tests on the state’s database without the FBI’s permission and found" an inexplicably high number of partial matches.	The employee was not a “rogue”; she did not need the FBI’s permission to use a state database this way; the results were presented on behalf of the state laboratory at an International Symposium on Human Identification. They are not especially anomalous, but largely a consequence of trawling for partial matches among all possible pairs of profiles in a database that includes close relatives. See, e.g., David H. Kaye, Trawling DNA Databases for Partial Matches: What Is the FBI Afraid Of?, 19 Cornell J. L. & Public Pol'y 145 (2009).

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