In the Physicians Health Study over 22,000 male physicians were randomly assigned to two groups. One group took an ordinary aspirin tablet every other day while the other group took a placebo. The study was double-blinded. After a number of years, the number of heart attacks was recorded for each of the subjects. For the aspirin group the number of attacks per 1,000 doctors was 9.42. The corresponding rate for the placebo group was 17.13.

(a) Is this an experiment or an observational study? Explain your choice.

(b) What are the explanatory and the response variables in this example? Indicate whether each is qualitative or quantitative.

(c) A critic of the study acknowledged that the aspirin group had a substantially lower heart attack rate than the placebo group, but suggested that the difference may be explained by the fact that the aspirin group may have been significantly healthier to begin with than the placebo group. Is the critic correct? Please explain.