Maths 101.

I've posted before on the dangers of misusing and misunderstanding mathematical data. In a recent case a defendant was convicted on the basis of DNA evidence. The prosecution maintained that DNA had been found at the scene of the crime, and that there was only "one chance in a billion" that it was not that of the defendant. How they arrived at this figure was not explained, and I have the feeling that it was just plucked out of the air for effect, but even if we take it as accurate, there are more than 6.5 billion people in the world, which means that there are probably 6 or 7 people out there who would fit that DNA profile. So in the absence of any other evidence you could say that there is only a 15% chance that they have got the right person. It all depends on the way you look at it, doesn't it?