The federal judge out of Illinois admitted ballistics evidence despite the PCAST report because he considered estimated false-positive rates relatively low. Here's the critical passage on that score:
PCAST did find one scientific study that met its requirements (in addition to a number of other studies with less predictive power as a result of their designs). That study, the “Ames Laboratory study,” found that toolmark analysis has a false positive rate between 1 in 66 and 1 in 46. Id. at 110. The next most reliable study, the “Miami-Dade Study” found a false positive rate between 1 in 49 and 1 in 21. Thus, the defendants’ submission places the error rate at roughly 2%. The Court finds that this is a sufficiently low error rate to weigh in favor of allowing expert testimony. See Daubert v. Merrell Dow Pharms., 509 U.S. 579, 594 (1993) (“the court ordinarily should consider the known or potential rate of error”); United States v. Ashburn, 88 F. Supp. 3d 239, 246 (E.D.N.Y. 2015) (finding error rates between 0.9 and 1.5% to favor admission of expert testimony); United States v. Otero, 849 F. Supp. 2d 425, 434 (D.N.J. 2012) (error rate that “hovered around 1 to 2% ” was “low” and supported admitting expert testimony). The other factors remain unchanged from this Court’s earlier ruling on toolmark analysis.Using a 2 percent error rate could understate things: The error rates from the studies he cited ranged from 1.5 to 4.8 percent, so it could be twice that high (1 in 21). Still, I'm not surprised that some judges might consider an error rate of 1.5 to 4.8 percent acceptable. And the judge is surely right that the PCAST report provides a new basis for cross-examining experts and reduces the level of certainty about their findings which experts can portray to juries, so that's a plus.
OTOH, an erroneous ballistics match - and even though analysts can't use the word "match" any more, it's how jurors will inevitably view such testimony - will loom large for jurors and be highly prejudicial as evidence. So if you're the unlucky one in 49, one in 21, or whatever the real number is of people falsely accused by ballistics comparisons, jurors are likely to go with the so-called "expert" and the defendant is basically screwed.
Grits has estimated before that two to three percent of criminal convictions involve actually innocent defendants - not too different than the judge's error rate he considers allowable on ballistics. But that rate gets you to thousands of unexonerated people sitting in Texas prisons alone, with many more on probation, parole, and who have already completed their sentences. Given the volume of humanity which churns through the justice system, two or three percent is quite a significant number of people.
I'm curious as to Grits readers' opinions: How high a false positive rate is too high? Is forensic evidence that's 95 to 98 percent accurate good enough to secure a conviction "beyond a reasonable doubt" if that's the principle evidence against a defendant? At what error threshold should forensic evidence be excluded? Make your case in the comment section.