(35S) Conditional Probabilities - Bayes Formula - Printable Version +- HP Forums (https://www.hpmuseum.org/forum) +-- Forum: HP Software Libraries (/forum-10.html) +--- Forum: General Software Library (/forum-13.html) +--- Thread: (35S) Conditional Probabilities - Bayes Formula (/thread-2496.html) (35S) Conditional Probabilities - Bayes Formula - Gerald H - 11-25-2014 03:00 PM Bayes’ formula is useful for calculating conditional probabilities. If we set p(E) = A = .001 as the probability of a member of the population having a certain illness E, p(F|E) = B = .99 as the probability of a positive result for the illness test F if the member has the illness E, p(F|-E) = C = .05 as the probability of a positive result for the illness test F if the member does NOT have the illness E, a so-called false positive, then the formula D = A * B / ( A * B + ( 1 – A) * C ) can be used to calculate p(E|F) = D ie the probability of having the illness E if the test F is positive, which for the given data is .019