I'm unsure if the British system is equally flawed but it's an angle
which I had not previously considered.
This account is relating what's currently happening in the USA.
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Like fingerprints, DNA are very powerful and scientifically sound
evidence, when used to connect a known suspect to evidence found at the
scene of the crime. Jurors are easily persuaded to accept the DNA link for
someone who had already been suspected of a crime scene when told the odds
against a false identification are 1 in millions or billions.
But DNA is far less certain when you compare one sample against all of the
profiles in the database typically known as one-to-many. In that case the
chances that a match between a DNA sample -- especially an incomplete one
-- and a person in a DNA database could nab an innocent person has
different math. Very different math.
So if you have a probability of 1 in 1.1 million chance of people having a
certain sequence of DNA markers and you have a database of 550,000 people,
you have a 50% chance of making a match. That's great, if you know that
the perpetrator is in that database. But what it also means is that as you
start testing DNA profiles against more and more people, the chances that
you will match an innocent person to a DNA profile from a crime scene gets
higher.
>>
http://blog.wired.com/27bstroke6/2008/05/feds-to-collect.html
Exactly, you need other corroborating evidence before DNA evidence is even
worth a spit. Without that other evidence your DNA match is virtually
certain to latch on to an innocent person as well as the guilty party
(assuming he is in the database).
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