Debate on analyzing 'cold hit' DNA matches swirls in case

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Debate on analyzing 'cold hit' DNA matches swirls in case before California Supreme Court

A long-time scientific controversy centers on how to calculate the probability that such a match would be the result of coincidence.

One of the key issues in a case argued Thursday before the California Supreme Court is how to describe the significance of a "cold hit," when a criminal suspect is first identified through a DNA database search.

The question has been the subject of scientific controversy for more than 15 years and centers on how to calculate the probability that such a match would be the result of coincidence.

Though people's DNA is unique, genetic profiles are not. They are based on a sliver of markers on human chromosomes. Different people can share markers and in some cases even unrelated people share the same genetic profile by chance.

When other evidence has led investigators to a suspect, prosecutors can legitimately argue that the chance of such a coincidental "match" between DNA from the crime scene evidence and the suspect is remote. They cite a calculation called the random match probability: the chance that a person, selected randomly in one try from the general population, would happen to have a profile that matched the crime scene evidence. (This is sometimes called the "rarity" statistic.)

But when a database is searched, hundreds of thousands, sometimes millions, of comparisons are made to profiles within a database, increasing the odds -- sometimes significantly -- that a coincidental match will occur.

LA Times