  Stepped wedge designs (SWDs) are designs for cluster randomized trials that
feature staggered, unidirectional cross-over, typically from a control to a
treatment condition. Existing literature on statistical power for SWDs
primarily focuses on designs with a single treatment. However, SWDs with
multiple treatments are being proposed and conducted. We present a linear mixed
model for a SWD with two treatments, with and without an interaction between
them. We derive closed form solutions for the standard errors of the treatment
effect coefficients for such models along with power calculation methods. We
consider repeated cross-sectional designs as well as open and closed cohort
designs and different random effect structures. Design features are examined to
determine their impact on power for main treatment and interaction effects.
