How far should fingerprints be trusted?

The so-called "50K study" took a set of 50,000 pre-existing images of fingerprints and compared each one electronically against the whole of the data set, producing a grand total of 2.5 billion comparisons. It concluded that the chances of each image being mistaken for any of the other 49,999 images were vanishingly small, at 1 in 1097.

But Meagher's study continues to be severely criticised. Critics say that showing an image is more like itself than other similar images is irrelevant. The study does not mimic what happens in real life, where messy, partial prints from a crime scene are compared with inked archive prints of known criminals. [...]

He wrote that critics misunderstood the purpose of his study, which sought to establish that individual fingerprints are effectively unique - unlike any other person's print. "This is not a study on error rate, or an effort to demonstrate what constitutes an identification."

[...] From these Cole estimates that false matches occurred at a rate of 0.8 per cent on average, and in one year were as high as 4.4 per cent. Even if the lower figure is correct, this would equate to 1900 mistaken fingerprint matches in the US in 2002 alone.

But on CSI, the evidence is never ambiguous, and they always get a confession before the end of the episode! Real life is so unfair!


13 Responses:

  1. teferi says:

    Policing is hard, let's go shopping!

  2. The trade-off is that at least your real life isn't interrupted every 10 minutes for advertising.

    Oh, wait. Never mind.

  3. lars_larsen says:

    Why haven't criminals learned about gloves yet?

    I mean... really...

    • king_mob says:

      How do I put this? Your basic criminal did not vacate an MIT fellowship when he chose a life of crime. Haven't you ever heard the "I wore the juice" story?

      • lars_larsen says:

        Yes, but haven't they watched television?

        Wait, I guess they sold the TV for crack long ago.

      • jesus_x says:

        I had not heard the "But I wore the juice!" story before now. I must thank you for some incredible funny. I don't knwo how I missed this, because I was born, raised, and lived in Pittsburgh until 1998.

  4. greatbiggary says:

    Isn't the matching just about getting through the bulk of the data? I'd hope they'd at least make a printout to compare visually, or have the computer throw up a sweet soundfx filled comparison window screen that makes the fingerprints slide over one another with crosshairs ricocheting about, zooming in on like regions and running multiple "enhance" filters.

    Just trusting the computers match entirely would be like using Google's "Feeling Lucky" button. If that's the case, then those criminal punks just gotta ask themselves one question.

    • treptoplax says:

      It's worse than that, they often don't use the computers at all.

      Police send smudged partial prints to the lab, ask: Does this match fingerprint X?

      Technician eyeballs it, says, "Yes, that is a 5-of-7-point match" based on some crazy scale that varies wildly from lab to lab and has about as much basis in any kind of scientific testing as astrology.

      Ta da!

      • susano_otter says:

        Which is, presumably, why the fingerprints--and all the other evidence--is presented--along with arguments for and against--to a jury--under the supervision of a judge--for deliberation.

        Is it perfect? No. Are fingerprint comparisons a useful forensic tool? Yes.

        Anyay, think of it as "defense in depth", or multiple layers of redundancy.

        I wonder how often fingerprint evidence is challenged in court. If the comparison methods equate to astrology, and vary widely from lab to lab, it seems to me that we should see a lot of defense lawyers bringing in their own fingerprint experts to invalidate the prosecution's findings. Does this really happen, or do most lawyers consider it a dead end? Maybe fingerprints are reliable enough that investing defense resources in invalidating them is a losing proposition.

        Anyway, it's not like this research means we should throw out fingerprints as a valid source of evidence. It just means that we should probably not rely on fingerprints alone to convict (or acquit) a suspect.

        Oh, wait. We don't. So at least that's settled.

        Let's go shopping!

        • jwz says:

          See, this is another one of those cases where actually reading the article would have answered your question:

            ...challenging the US Department of Justice's long-standing contention that fingerprint evidence has a "zero error rate", and so is beyond legal dispute. Indeed, fingerprint examiners have to give all-or-nothing judgements. The International Association for Identification, the oldest and largest professional forensic association in the world, states in a 1979 resolution that any expert giving "testimony of possible, probable or likely [fingerprint] identification shall be deemed to be engaged in conduct unbecoming".
        • Well, and even then, they lie. In the book "The Evidence Never Lies", the author (did the original research on blood splater evidence) showed the court how the FBI faked the finger print, and how you could show it was a fake (ruler next to print nice and sharp, print fuzzy, no way to do that with the original by accident, only with a composite photo, and thus a blatant forgery).

  5. jayrtfm says:

    NPR had a couple of good interviews with Simon Cole, author of Suspect Identities: A History of Fingerprinting and Criminal Identification.