Quiz: The Probability of Having a Disease

A new super-resistent virus has emerged in Farawayistan. The Ministry of Health estimate 1 out of 1000 to be infected. After immense efforts, a very accurate test has been developed. If the test subject is infected, 99.9% of the time the test will come out positive. But if the subject is not infected, 1 in 1000 times the test will yield a false positive result.

A random person is tested, and the test comes out positive. What's the probability that he's infected?

Please, answer in the comments.


Anonymous said...

1/1000 because the test is only false so often. The rest is irelevant.

Anonymous said...

I think 10 to 1 or something. The rest is not irrelevant. If we know before the test that no one is sick, its sure not 1/1000, its 0/1000. :-)


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