Carryover and the two-period crossover clinical trial.

The use of the two-period crossover trial for comparing two noncurative treatments in patients with a chronic disease is appealing since it eliminates the between-subject variability. However, the possibility of the existence of carryover effect leads many authors to advise that a parallel design be used whenever carryover is suspected. We examine this advice and quantify the degree of carryover required to make the parallel design preferable …