"60 Minutes" had a big piece on the Dallas "Innocence" project. I suspect we have gone overboard with DNA matches.
Bayes' Theorem and DNA Database Searches
By Radley Balko on Innocence
Via the comments to this Eugene Volokh post, it looks like the Ninth Circuit has just thrown out (pdf) a guilty verdict over precisely the problems with predicting odds when doing cold DNA database searches that we discussed earlier this week. Excerpt from the opinion:
Here, [DNA expert Renee] Romero initially testified that [defendant Troy Don Brown]'s DNA matched the DNA found in [rape victim Jane Doe]'s underwear, and that 1 in 3,000,000 people randomly selected from the population would also match the DNA found in Jane's underwear (random match probability). After the prosecutor pressed her to put this another way, Romero testified that there was a 99.99967 percent chance that the DNA found in Jane's underwear was from Troy's blood (source probability). This testimony was misleading, as it improperly conflated random match probability with source probability. In fact, the former testimony (1 in 3,000,000) is the probability of a match between an innocent person selected randomly from the population; this is not the same as the probability that Troy's DNA was the same as the DNA found in Jane's underwear, which would prove his guilt. Statistically, the probability of guilt given a DNA match is based on a complicated formula known as Bayes's Theorem, see id. at 170-71 n.2, and the 1 in 3,000,000 probability described by Romero is but one of the factors in this formula.
Once again, it's worth noting that if other evidence points to a suspect, and you then get a match to your suspect after running the crime scene DNA against a database, you can be reasonably certain of guilt. I'm just wary of using cold matches as the starting point of an investigation. Precisely because many people misunderstand the fairly high odds of false matches with large databases, you run the risk of the investigation becoming more about finding proof that the match committed the crime than about investigating who committed the crime. The problem grows when you're talking about decades-old cases where evidence has degenerated, witnesses have died, and records may or may not still be around. theagitator.com |
