Factorial Trials
Factorial clinical trials test the effects of two or more therapies using a design that can estimate interaction between therapies (Piantadosi 2017). (This chapter revises, updates, and expands upon reference (Piantadosi 2017)) A factorial structure is the only design that can assess treatment interactions, so this type of trial is required for those important therapeutic questions. When interactions between treatments are absent, which is not a trivial requirement, a factorial design can estimate each of several treatment effects from the same data. For example, two treatments can sometimes be evaluated using the same number of subjects ordinarily used to test a single therapy. When possible, this demonstrates a striking efficiency. For these reasons, factorial designs have an important place in clinical trial methodology, and have been applied in a variety of setting, but in particular in disease prevention.
