Suppose an illness that can affect 1% of the people. Also assume that there is a test for that illness, that gives the correct result 99% of the times.

If you take that test, and receive a POSITIVE result, should you worry much?

If you take it again, and once more get a POSITIVE, should you worry then?

How many consecutive POSITIVEs would you have to get in order to be sure that the chances of a wrong diagnostic are 1 in a million?

(In reply to This is a lousy example, but... by Penny)

...hence Charlie's reference to psychology.

The ELISA test for HIV is 99.5% accurate. The approximate percentage of infected persons in the U.S. is around 0.4%, or 1/250. With these numbers, there's only a 44% chance you actually have HIV if you get a positive result. This is why there is a heavy reliance on re-testing.

Edited on January 3, 2005, 8:28 pm

Posted by Eric on 2005-01-03 20:25:07