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DNA Cold Hits

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"DNA matches aren't always a lock" is the headline of this LA Times story by Jason Felch and Maura Dolan. Cases where a suspect is identified by a "cold hit" database search involve different probability considerations than the typical case where a person already a suspect based on other evidence is confirmed as the perpetrator by a DNA test.

By analogy, if you buy a lottery ticket for a drawing, the chances that your ticket will win are extremely small. However, the chances that somebody will win from that drawing are often pretty good. If the chance that a person would match a profile if selected at random are one in a million and the database searched for that profile has millions of people, then there is a decent chance that an innocent person will be a random match. Of course, with fresh samples and modern techniques, the random match probability is 1/quadrillions, not millions, so the chance of random false positive remains very small. With old, degraded samples and only a few markers available, though, the chance of a false positive match is significant.

Is this story accurate? I would be interested in hearing from DNA experts.

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I'm far from an expert (in fact I'm merely a law student who is working at a district attorney's office) but we litigated this issue quite a bit before a cold hit case we just did and I wrote most of the motions. The first part of the statement is true, the statistics involved in a cold hit are very different than random match probability. In fact in our case, the statistics from the cold hit alone were 1 in 135 males which is clearly far away from being anything useable. However, after the initial hit, the DNA analysts do a standard comparison, this is the statistic which is used at trial. In our trial, using random match probability comparing the defendant to the sample from the victim and using the entire population as the database, the statistical occurrence of the DNA was 1 in 7.9 million Caucasian males.

In short, the odds of a false positive on the initial data mining (for lack of a better word) are quite high but the odds of that match being the only comparison done are none existent.

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