The FBI, the New York-based Innocence Project and the National Association of Criminal Defense Lawyers are examining nearly 3,000 cases nationwide in which the FBI may have misused microscopic hair comparison.

The review so far found statements and findings that “exceeded the limits of the science” in more than 90 percent of the cases. The errors fall into three broad categories:

Claiming a ‘match’

What they did: Examiners stated or implied that the evidentiary hair could be associated with a specific individual to the exclusion of all others.

Why it was wrong: Absent DNA testing, hairs are not unique enough to be associated with one person, even by looking at them under a high-powered microscope.

Claiming a statistical weight

What they did: Examiners assigned a statistical weight, probability or likelihood that the questioned hair originated from a particular source.

Why it was wrong: No such weight can be assigned because no one knows how many people have microscopically identical hair.

Citing experience to bolster findings

What they did: Examiners cited statistics such as the number of hair cases they or the FBI lab had handled to bolster the findings.

Why it was wrong: Unlike DNA, there is no database of hair profiles. Analysts cannot memorize every hair they have ever examined. And comparing vast numbers of hairs — even billions — does not change the fact that an unknown number of people have hair that looks identical.

Sources: National Association of Criminal Defense Lawyers; FBI; Skip Palenik, Microtrace LLC.