A scanner to detect terrorists

http://news.bbc.co.uk/2/hi/uk_news/magazine/8153539.stm

If 3,000 people are tested, and the test is 90% accurate, it is also 10% wrong. So it will probably identify 301 terrorists - about 300 by mistake and 1 correctly. You won't know from the test which is the real terrorist. So the chance that our man in the mac is the real thing is 1 in 301.