Placebo Effect

Abstract A placebo effect is conceptually defined as the beneficial effect associated with an intervention that does not include the presumed active ingredients. It can be operationally defined as the positive difference between the results of a placebo condition and a no‐treatment condition in a randomized trial. Placebos play a crucial role as a control in clinical medication trials, but discussion remains regarding their effect sizes and possible working mechanisms. As a confounding factor, a placebo effect is considered as a specific type of reactivity effect that should be distinguished from other confounding factors like history, spontaneous recovery, or statistical regression.