# A First Course in Probability (8th) step-by-step solutions: Chapter 3 Problem 49P

Prostate cancer is the most common type of cancer found in males. As an indicator of whether a male has prostate cancer, doctors often perform a test that measures thelevel of the prostate-specific antigen (PSA) that is produced only by the prostate gland. Although PSA levels are indicative of cancer, the test is notoriouslyunreliable. Indeed, the probability that a noncancerous man will have an elevated PSA level is approximately 0.135, increasing to approximately 0.268 if the man doeshave cancer. If, on the basis of other factors, a physician is 70 percent certain that a male has prostate cancer, what is the conditional probability that he has thecancer given that:

(a) the test indicated an elevated PSA level?
(b) the test did not indicate an elevated PSA level?

Repeat the preceding calculation, this time assuming that the physician initially believes that there is a 30 percent chance that the man has prostate cancer.