When studying treatments for non-curable chronic diseases, including asthma, angina pectoris, hypertension, epilepsy, migraine etc., the crossover design, in which each eligible patient is randomly assigned to receive more than one treatment according to a pre-determined treatment sequence, has been often proposed to improve power or save the number of patients needed for a parallel groups design. This is because each patient serves as his/her own control in a crossover trial and thereby, we can eliminate the variation of responses between patients when comparing treatments. The simplest crossover design is the AB/BA design, in which patients are randomly assigned to either the AB group in which patients receive treatment A first and then crossover to receive treatment B, or the BA group in which patients receive treatment B first and then crossover to receive treatment A. Because of its simplicity, the AB/BA design, also called the simple crossover or 2 x 2 design, has accounted for a large proportion of crossover trials used in practice. Although the crossover design is of use, there are limitations due to its design features.
Author(s): Kung-Jong Lui