When conducting a double-blind clinical trial to evaluate a new treatment, the investigator is faced with the problem of what to give the control group. If there is no existing acceptable and [Page 1034]beneficial treatment against which the new treatment can be compared, the reasonable approach would be to give the control group no treatment at all, but this raises concerns that subjects may have hidden expectations or preferences that could influence the outcome. One solution is the use of a harmless “sham” treatment that is similar in all aspects such that the subjects and those administering the treatments cannot determine whether a subject is in the study group or the control group. This sham treatment is called the placebo. This entry discusses historical usage, ...