Performance enhancing drugs in athletes is a big issue and has been around for some time. One estimate is that 5% of all athletes use drugs (D) which means 95% are clean (C). In professional sports athletes are tested regularly, and some athletes who are clean test positive (P), and some who use drugs test negative (N) - false positives and false negatives. In relation to the reliability of testing it is estimated that 3% of Clean athletes test positive and 8% of Drug Takiing athletes test negative. P(P/C) = 0.03 and P(N/D) = 0.08. Note this implies P(N/C) = 0.97