The placebo defect
Suppose a clinical trial randomizes 100 patients to receive an experimental drug in the form of pills and an equal number of patients to receive identical pills except that they contain no active ingredient, that is, placebo. The results of the trial are as follows: 60 of the patients who received the experimental drug improved, compared to 30 of the patients who received the placebo. The drug clearly works better than the placebo. [1] But 30% of the patients who received the placebo did get better. There seems to be a placebo effect, right? Wrong. The results from this trial provide no information about whether or not there is a placebo effect. To determine whether there is a placebo effect you would need compare the outcomes of patients who received placebo with the outcomes of patients who received no treatment. And not surprisingly, trials with a no-treatment arm are quite rare. But there are some. In a landmark paper published in the New England Journal of Medicine in 2001 ( fre...