26. The concern over drug use among teens has prompted some schools to consider mandatory drug testing of its students. However, schools should be warned that drug tests, while generally reliable, are not perfect. Sometimes a person who does use drugs gets a negative result (a false negative). Sometimes a person who does not use drugs tests positive (a false positive). Companies that produce tests for drugs usually provide information on two characteristics of their test: , the probability that their test correctly identifies a drug user, and , the probability that it correctly reports the absence of drugs.

Suppose that a large high school is considering a mandatory drug testing program. The particular test under consideration correctly identifies 95% of the users and correctly reports the absence of drugs in 90% of the nonusers .