DNA testing

Obvioulsy we all hope that the brutal killer of 18 year old Sally Anne Bowman is caught soon and brought to Justice.

But the police plan of taking DNA from 2500 local men does seem to have one basic logical flaw. Surely the murderer won’t volunteer? If this fails, what is the police’s next plan: to put an advert in the local paper asking the murderer to hand themselves in exchange for free chocolate?

There are two very real problems. One of which is a misunderstanding by the public, and the police, of how reliable DNA testing is to gain unique identification . One of the misguided people, Lecturer Jeremy Johnson, 31, who volunteered to give their DNA told the BBC: "I just wanted to get myself eliminated and the test will definitely help catch the killer. There's only a one in a billion chance you'll be wrongly convicted.”

One in a billion? Well according to Professor Ross Anderson a world famous security specialist at Cambridge University, one white person in 120 has an identical twin, with excatly the same DNA. Some twins may be adopted and not even know that they have a clone out there!

Also, let us assume human error in testing of even as low as one in 10000, this still leaves a reasonably high chance of a false identification. This is the famous "birthday paradox" - that the chances of there being two people who share the same birthday in group of 20 or so people is very high. If there is an error of testing, then the chances of an innocent person being identfied is much higher than non-statisticians might believe.

Testing errors and shared DNA lower the odds to a lot lot more than one in a billion.

Remember that fingerprint evidence used to be argued as infallible, but a system that used to compare a set of prints manually with 57 local burglars may be pretty reliable, but when you start using a national database, the chances of false matches becomes increasingly high.

But the second problem is that juries place excessive trust in DNA evidence as being conclusive, which means it is very tempting for police (who may be genuinely convinced of someone’s guilt) planting DNA evidence. Having a database of samples just makes it easier for them to fit people up. Or even for a criminal to frame you.