In Exercises 21–28, determine whether the study is an experiment or an observational study, and then identify a major problem with the study.

Physicians’ Health Study The Physicians’ Health Study involved 22,071 male physicians. Based on random selections, 11,037 of them were treated with aspirin and the other 11,034 were given placebos. The study was stopped early because it became clear that aspirin reduced the risk of myocardial infarctions by a substantial amount.

Out of 22,071 male physicians, 11,037 were given aspirin, and the remaining 11,034 were given placebos. The study was conducted on the two groups to assess the effect of aspirin in reducing the risk of myocardial infarctions by some amount.

Anobservational studyis a study used to measure observations and effects of the desired treatment on a set of unitswithout any intervention of the experimenter.

Anexperimental studyis a study used to examine the observations and effects of the desired treatment on a set of unitswith the proper intervention of the experimenter. Treatments are selectively applied to two or more subgroups of the entire population of units.

A dummy of the original drug withno real medicine is called aplacebo. It is used to identify the effectiveness of a real drug by administering one group of subjects with the real drug while the other group receives the placebo. In this manner, the psychological bias induced is avoided.

The given study deals with examining the effect of aspirin on the occurrence of myocardial infarctions by giving one group of physicians the real drug while the other receives a placebo. Thus, it is an experimental study.

Since the set of subjects for this study consists of only male physicians, the results cannot be generalized for the entire population, which includes female individuals as well. As the sample considered here is not representative of the entire population, to claim that aspirin significantly reduces the effect of myocardial infarctions is inaccurate for the entire population. Also, the study was stopped early, so the result of the study was not trustworthy.