Suppose we assume that 5% of people are drug users. A drug test is 95% accurate (i.e., it produces a correct result 95% of the time, whether the person is using drugs or not). A randomly chosen person tests positive. Is the person highly likely to be a drug user?

Marilyn's answer: Given your conditions, once the person has tested positive, you may as well flip a coin to determine whether she or he is a drug user. The chances are only 50-50. But the assumptions, the make-up of the test group and the true accuracy of the tests themselves are additional considerations.