The Physicians’ Health Study II was a randomized, double-blind, placebo-controlled study designed to “test alternate-day beta-carotene, alternate-day vitamin E, daily vitamin C, and a daily multivitamin in the prevention of total and prostate cancer, cardiovascular disease, and the age-related eye diseases, cataract and macular degeneration.” The subjects were 14,641 U.S. male physicians (older than 50 years in the year 1997) who were randomly assigned to take twice-daily beta-carotene, twice-daily vitamin E, daily vitamin C, daily multivitamin, 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.