Some data points

OK, so by chance we have some real data to put into the sums in this post. The head of MI5 has just announced that we should all be very scared, because he reckons there may be 2,000 people in Britain who pose a threat to national security because of their support for terrorism.

So let's run the Terroriser. 59 million people; 2,000 terrorists. So there's a 0.0034% chance of any given citizen being a terrorist. Remember that the Terroriser will catch 99 per cent of the real terrorists - so that's all but 20 terrorists. Now, the Terrorist will also miss 98 per cent of the non-terrorists - but that means we'll get some 1,180,000 false positives. 1,980 terrorists plus 1,180,000 false positives = 1,181,980 suspects. (1,980/1,181,980)x100=0.1675155. There is a 0.167 per cent chance that any one of the suspects is a terrorist.

And there are still 20 terrorists out there; easily enough for a major terrorist attack. Now consider this hilarious report; apparently the FBI mined supermarket sales figures in the hope that sales of falafels would indicate the presence of Iranian terrorists! As well as, ah, Israelis, presumably. Note the involvement of half-arsed fearmonger Steven Emerson, and also old TYR butt Yossef Bodansky.