This is a slide from a lecture I had: http://www.geocities.com/thesquarerootoftwo/bayes.JPG What we're looking at is a test for the presence of a bomb, like something airport screeners might use. When the probability of the bomb, P(B) is figured, it looks like the calculation is done by taking the total number of times the bomb was present and dividing it by the total number of times the bomb was absent. Is this correct? I was thinking you would take the total number of times the bomb was present and divide by (the total number of times absent + the total number of times present). I have the same question about P(T) which I think is the probability of a positive test. But I don't know jack about Bayes Theorem, so thanks for any help!