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 re: solution by Penny)

Well, I also got 50%.

I did the same thing as Charlie - out of 10,000 people, there are 100 people who have the disease. Of those 100, 99 are correctly diagnosed. There are 9900 people who are not infected, yet 99 of them (1%) are diagnosed as having the disease. So, out of 10,000 people, there are 198 people who are diagnosed as being positive yet only 99 actually are positive. So, there's exactly a 50% chance of being a carrier.

Posted by Eric on 2005-01-03 20:05:06