The UK National DNA Database currently has about 3 million people’s biometric information stored in it.

If DNA recovered at crime scene has a 1 in 50,000 chance of matching someone, what is the probability that a match will be found even if the actual perpetrator’s information isn’t in the database?

1 – (1 / 50,000) ^ 3,000,000 = 99.9999%

Basically, it’s a certainty. Someone’s profile will be returned falsely.

That’s 1 in 50,000 chance though. That’s a weak test of the sort you get from old, broken-down DNA. What if it was a fresh sample, and the chance of a random match was 1 in 100,000,000 (a hundred million)?

1 – (1 / 100,000,000) ^ 3,000,000 = 2.9554%

That’s approximately double the chance of winning £10 on the National Lottery (1.7544%).

How come they never mention this on C.S.I?