March 16, 2010

It’s unusual to see this

Filed under: Uncategorized — ubikcan @ 6:38 pm

point made, so credit where it’s due:

Lindsay Beyerstein describes a potential new tool for criminal investigators, “traces of bacterial DNA [that] can be used to link people to objects they’ve touched.”

The technology isn’t ready for prime time yet. So far, the method has only proved 70% to 90% accurate. Much more testing will have to be done before this kind of evidence gets anywhere near a courtroom.

I certainly hope more testing is done, because this seems like a technology that comes fully loaded with the possibility of serious mathematical errors. For example, suppose I find a dead body, a murder weapon, and on the weapon traces of bacterial DNA. Then I pluck a random person off the streets of Washington DC and the bacteria on his finger matches according to a test that’s 90 percent reliable.

To see why this doesn’t mean what it might seem, click here (and notice that improving the test doesn’t change the issue all that much).



  1. The issue is epistemological, not empirical. Nobody is saying that this test could, or should, be considered proof beyond a reasonable doubt in court–even if the technology improves a great deal. In a way, DNA evidence has spoiled the general because it invites the false assumption that every valid forensic tool definitively picks out a single individual.

    Most forensic evidence is neither perfectly sensitive nor perfectly specific. Any given test result can be explained away, but if a test is part of a mutually supporting network of forensic and other evidence, it can contribute to establishing the burden of proof.

    Blood type evidence is not considered dispositive on its own, but it is still useful within certain bounds.

    Comment by Lindsay Beyerstein — March 16, 2010 @ 10:16 pm

  2. Thanks for the comment. You are probably right that this test cannot be used on its own in court beyond a reasonable doubt.

    Here’s the mathematician John Allen Paulos, writing in his column a few years ago on false positives:

    “Even if the probability that the purported terrorist profile is accurate were an astonishing 99 percent (if someone has terrorist ties, the profile will pick him or her out 99 percent of the time, and, for ease of computation, if someone does not have such ties, the profile will pick him or her out only 1 percent of the time), most of the hits would be false positives.

    For illustration, let’s further assume that one out of a million American residents has terrorist ties — that’s approximately 300 people — and the profile will pick out 99 percent, or 297 of them. Great. But what of the approximately 300 million innocent Americans? The profile will also pick out 1 percent of them, “only” 3 million false positives, innocent people who will be caught up in a Kafkaesque dragnet. ”

    More here.

    Comment by ubikcan — March 17, 2010 @ 8:07 am

RSS feed for comments on this post. TrackBack URI

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

Blog at WordPress.com.

%d bloggers like this: