The Ceiling Effect, the Floor Effect, and the Importance of Active and Placebo Control Arms in Randomized Controlled Trials of an Investigational Drug

Randomized controlled trials (RCTs) of a new investigational drug often include active as well as placebo control arms. The active arm, comprising an approved treatment for the indication under study, along with the placebo arm, are together required to establish assay sensitivity; if the active treatment outperforms placebo, as expected, the results of the RCT can be further interpreted, but if the active treatment is no better than placebo (such as because of ceiling or floor effects), the RCT is a failed trial. The concepts involved are explained from scientific and ethical perspectives.