Researchers conduct a study to determine whether a medication based on fish oil known as omega-3 carboxylic acids is effective in reducing the risk of cardiac events in people already at risk. In the study, 13,078 patients are randomly chosen to receive either omega-3 carboxylic acids or a placebo of corn oil daily.

Researchers compare the number of cardiac events of the two groups over similar follow-up periods of time.

In Exercises 19 and 20, determine whether the study is an observational study or an experiment. Explain.