Question: Suppose 100 hypertensive people are given an antihypertensive drug and the drug is effective in 20 of them. By effective, we mean their diastolic blood pressure is lowered by at least 10 mm Hg as judged from a repeat measurement one month after taking the drug.

Suppose we know that 10% of all hypertensive patients were given a placebo will have their DBP lowered by 10 mm Hg after one month. Can we carry out some procedure to be sure we are not simply observing the placebo effect?