Three-period crossover designs for two treatments.

The use of three periods in the two-treatment crossover design for clinical trials is considered. It is proposed that a series of such trials in a particular therapeutic area may establish the relevance of the crossover design in that area. Treatment sequences to be used in three-period two-treatment trials are discussed. Two possible designs which allow carryover effects to be tested against the within-subject variability are compared. A design involving four treatment sequences is recommended.