IN a famous experiment to determine the efficacy of aspirin in preventing hear attacks, 22,000 healthy middle-aged men were randomly divided into two equal groups, one of which was given a daily dose of aspirin and the other a placebo that looked and tasted identical to the aspirin. The experiment was halted at a time when 104 men in the aspirin group and 189 in the control group had had heart attacks. use these data to test the hypothesis that the taking of aspirin does not change the probability of having a heart attack.