Despite what you may have heard, randomized trials are not always free of confounding and selection bias. Randomized trials are expected to be free only from baseline confounding but not from postrandomization confounding and selection bias (1). In this commentary, we describe the settings in which postrandomization confounding and selection bias emerge in randomized trials, discuss the shortcomings of intention-to-treat analyses to handle these biases, and direct readers to more appropriate methods.

The neglect of postrandomization confounding and selection bias in randomized trials is the historical consequence of the fact that many early trials were short, small, double-blinded, tightly controlled ...