This is a thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. The previous "stupid" questions thread went to over 800 comments in two and a half weeks, so I think it's time for a new one.

This is a thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. The previous "stupid" questions thread went to over 800 comments in two and a half weeks, so I think it's time for a new one.

Another stupid question about Bayes Theorem.

Let's say I go to the doctor and take some sort of screening test for cancer. Only 1% of the population has this cancer, and this test has an 80% success rate and 10% false positive rate. The test says I have the cancer, so this means that I'm now at approx. 7% probability that I have the cancer. If I go to a different doctor's office the next day and take the same test, am I updating on the original 1% or am I updating on the new 7%?

You update on the 7%, but the conditional probabilities of getting a positive result if you have and not have cancer respectively have also changed so that another positive test gives you less information than what it took to bring you from 1% to 7 %. See what ygert said.