LAUSR

The placebo effect is present in all experiences of interventions, with the magnitude of the effect potentially influenced by several factors.1 The observed summary measure of the primary outcome (eg, mean/ median) in each treatment arm or intervention is thus a combination of the real therapeutic effect, the placebo effect, and the natural progression of the condition since treatment initiation (Box 1). The magnitude of each of these components may vary within both person and group. Placebo controls are intended to control for the placebo effect, but where the placebo effect is not equal across interventions, this is unlikely the case and the magnitude of the therapeutic effect remains unknown. To control for the placebo effect, a placebo control should have no specific therapeutic effect on outcomes of interest and be perceived as real and identical to the primary intervention.2 These placebo criteria are often not possible to achieve for all interventions (eg, lifestyle interventions). Despite this, several studies have compared such interventions with socalled placebo controls that do not meet these criteria, potentially producing misleading results.