Made out of people

The use (and abuse) of statistics

Spanish investigators found a piece of evidence in a white van 20 miles from the blast that they did not reveal: on a plastic bag containing bomb materials, investigators found a "perfectly formed" fingerprint they couldn't identify, a Spanish official tells Newsweek. Spanish police didn't find a match for the print, but when the FBI ran it through its archive, the computer unexpectedly logged a hit: the mystery print, U.S. authorities say, belonged to Brandon Mayfield, a small-time lawyer who lived in Portland, Ore. Mayfield had been fingerprinted years earlier, when he served in the U.S. Army. ( Newsweek )

Now, I don't know the accuracy of fingerprint recognition. Clearly, though, there's some level. (This website advertises a system with 1 in 2.5 million accuracy. Another article¹ cites a number more like 1 in 16 million.)

I completely believe that if you have a suspect who you have good reason to believe was at a crime scene, than one in a few million is a great narrower. But this did the reverse technique: it filtered through the entire US fingerprint database, and it found a hit.

Now, remember that if you are "one in a million," there are 293 of you in the USA--and there are 6,367 of you in the world (according to the US census bureau).

There's no other evidence on this man. His passport is expired, and there's no record that he's left the country. He's being held as a "material witness" indefinitely. Which means--to my reading of the article--that he's being held on the basis of a statistical measure.

Update 5/14/04. The Newsweek article gives a different degree of detail:
bq. Law-enforcement officials today provided few details about the evidence against Mayfield, but said the alleged presence of physical evidence tying the man to the Madrid bombing made it an extremely serious matter. Sources said Mayfield's fingerprints were found on a bag containing bomb material connected to the Spanish attack. But officials said considerable uncertainty remained about Mayfield's role.

Crooked Timber also asks about the justice of the material witness law, especially as it links to this issue

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¹ # S. Pankanti, S. Prabhakar, and A. K. Jain, On the Individuality of Fingerprints, IEEE Transactions on PAMI, Vol. 24, No. 8, pp. 1010-1025, 2002