ABSTRACT
In a crossover design, the subjects act as their own controls. For example, in a placebo-controlled double
crossover trial with two arms, at the time of crossover, the subjects who had been receiving the placebo during the rst part of the study now cross over to the active treatment, while the subjects who had been receiving the active treatment now cross over to the placebo. In contrast, in a placebo-controlled single crossover trial with two arms, at the time of crossover, the subjects who had been receiving the placebo during the rst part of the study now cross over to the active treatment, while the subjects who had receiving the active treatment continue to receive the active treatment for the rest of the duration of the trial. A trial is double-blind if both the subjects and the observers are blinded to group allocation.
