NO2ID

http://www.guardian.co.uk/politics/2010 ... base-crime

Letters
Prosecutor's fallacy

* The Guardian, Tuesday 6 April 2010

In your report (31 March) of Tony Blair's speech in Sedgefield he is quoted as saying that the DNA database "provides incontrovertible evidence of innocence or guilt". What the DNA database can do is to show that there is, or is not, an incontrovertible link between a suspect and a crime scene or individual.

...

RE Ardrey, AR Allan and PG Ashton
Directors, Triple A Forensics

I would venture to suggest that it is the *samples* which do this, not the database - am I right in thinking that when they narrow down their list of suspects the samples are re-processed to a higher accuracy than the profiles on the database? Or at the very least to confirm that the database isn't wrong?

DrWibble



Not so far as I am aware. The system I was familiar with looked at a particular sequence along the DNA strand and, to confirm a match necessitated a string of ten identical "bases" (adenine, thymine, cytosine and guanine), plus a sex identifier. It had been calculated that there is only one chance in a billion of two people having the identical profile. Nearly 19 out of every 20 people in the UK will have a DNA profile which is unique in the country (including the 90% of the population who are not on the database). Even where a duplicate does occur, the odds that an identical profile is found at the scene of a crime which matches a local individual with a criminal past must be infinitesimally small. In spite of the minuscule odds of a false match, if someone insists that the DNA could not be theirs, the police will have to produce corroborative evidence in addition to the DNA. Mostly, that's not too much of a problem.

Stu

Stu - thanks for that - I had been sure that ther was some kind of 'higher resolution' processing that was done prior to court for confirmation.


Kind of what I'm getting at, I suppose - people using phrases like "incontrovertible link" and yet corroborative evidence is still required. I won't dispute that it's strong evidence, just the assertions of absolute certainty that we keep seeing.

That said, this is probably somewhat borderline off-topic-ness so I'll stop there...

And of course whether the presence of DNA at the scene is strong evidence of guilt or no evidence at all is always going to depend on the facts of the particular case. It could easily be the latter if a suspect has an innocent reason to have been on the spot.

Blair is a disgrace to his inn.