“In the fifty-seven-month study, whose participants were all male physicians, 104 of those who took aspirin had heart attacks, as compared with 189 heart attacks in those who took only a sugar pill. This means ordinary aspirin reduced the heart attack risk for health men by 47 percent. At least seven long-term studies of more than 11,000 heart attack victims have shown that one-half of one aspirin per day can reduce the risk of a second attack by up to 20 percent.” Identify the casual hypothesis at issue.