blog.world3.net

An interesting thread started up on a /. article about how handwriting style “fingerprinting” is easily fooled. Since I made some of the posts, I’m mirroring it here:

amimojo:

This should not really come as a surprise to anyone. Like all evidence that has to be interpreted, the interpretation can be flawed.

Shows like CSI have computers getting an exact match on fingerprints and DNA, but the real world is not like that. Fingerprint matching is entirely subjective and the print recovered from a crime scene is rarely a nice clean one like they show on TV. DNA often has to be manipulated before a match can be made (due to the sample found at the scene being too small or of poor quality) and even then it often matches more than one person.

Even when you do get a match, it’s not proof that someone was at a specific place because DNA and fingerprints can easily be transferred. Someone broke in to my car a few years ago and despite there being fingerprints the police decided not to prosecute because they were on the outside of the car and the accused could just claim he lent on it on his way home from the pub.

There have been a few cases where fingerprint and DNA evidence have been challenged in the UK courts and shown to be unreliable, with innocent people spending years in jail before being cleared. Yet, the police seem to have started asking for everyone in the area of a crime to “volunteer” their DNA. Presumably if you don’t “volunteer” you become a suspect.

The idea that handwriting is any more unique than those two and at all reliable is laughable.

abigsmurf:

There was a good article here (or possibly some other social news type site) about the inherent flaw in DNA databases and the weight given to DNA evidence.

The theory goes like this: the chances of getting a false positive on a part sample are something like 1/50million. You have 50 million people on the database. This means You’d expect a false positive on every search. If you’re unlucky enough to live close enough to a crime to have committed it, you could easily find yourself in court.

You’ll then have to defend yourself based on a 1 in 50 million probability to a jury who won’t understand the statistics. If you haven’t got a solid alibi, it would be a tough thing to do.

There’s probably a good Terry Pratchett quote about 1 in a million chances to be used here.

amimojo:

An excellent point well made.

There is also danger of a match being made on another member of your family, but you being the one somehow tied to the case (in the same city or something) and so you get arrested. Siblings have close enough DNA that such matches can apparently be made.

I question the “1 in 50 million” statistic too. It’s far too simplistic, as there are different ways of collecting and matching DNA. Also, so-called experts have been wrong about this sort of thing in the past. Remember that poor woman who spent years in jail because some idiot said that there was a “1 in a million” chance of having three children all die of cot-death?