The prosecutor's fallacy
The Supreme Court reversed the 9th Circuit again in McDaniel v. Brown. As with a number of other cases the 9th circuit found for a defendant in a writ of habeas case, and reversed the conviction. That seems to be the recipe for an almost automatic reversal. The opinion was unremarkable except for the discussion of a concept the Court acknowledged as the prosecutor's fallacy.
Mr. Brown was charged with sexual assault. The victim could not identify him, and the evidence was all circumstantial; the type where it could support innocence just as easily as guilt. The most compelling evidence was DNA recovered from sperm on the victim's panties. And it was the DNA evidence that was the focus of the writ proceeding.
Mr. Brown lived with his brother, and there was another brother that also knew the victim. They all lived in the same trailer park, so it was obvious that there would be an issue as to whether the DNA could be attributed to one of the brothers. The argument was over probabilities; according the State's expert, the probability that another person from the general population would have the same DNA profile was 1 in 3,000,000. The defense expert expert said it was more like 1 in 6,500.
The prosecutor's fallacy is the assumption that the random match probability is the same as the probability that the defendant is the source of the DNA sample. In other words, you cant take that the above statistic and say the probability that someone other than the defendant committed the offense was 1 in 3,000,000; or that there is a 99.9% chance that the defendant is guilty.
In the end, the interpretation of statistics was not critical to the decision. Even with the lower estimates, the evidence was still sufficient to support the conviction. The case does point out the necessity to challenge State's evidence though. You may need a statistician to do it, but that may be necessary to explain what the numbers mean. After all, 1 in 6,500 is a heck of a lot easier to work with than 1 in 3,000,000.
