The Physicians’ Health Study I was a very large, randomized, double-blind, placebo-controlled study designed to “test the effects of low-dose aspirin and beta-carotene on cancer and cardiovascular disease.” The subjects were 22,071 U.S. male physicians (aged 40–84 years in the year 1982), who were randomly assigned to take a twice-daily low-dose aspirin or beta carotene, or a placebo version of one of these. 

a. Is this an experiment or an observational study? Explain how you are deciding. 

b. Explain what it means for a study to be “double blind.” Also, explain the purpose of using double blinding in this study. 

c. What does it mean for this study to be “placebo controlled” and what is the purpose of a placebo? 

d. Explain how you would go about assigning the subjects to the groups.