  A popular design choice in public health and implementation science research,
stepped wedge cluster randomized trials (SW-CRTs) are a form of randomized
trial whereby clusters are progressively transitioned from control to
intervention, and the timing of transition is randomized for each cluster. An
important task at the design stage is to ensure that the planned trial has
sufficient power to observe a clinically meaningful effect size. While methods
for determining study power have been well-developed for SW-CRTs with
continuous and binary outcomes, limited methods for power calculation are
available for SW-CRTs with censored time-to-event outcomes. In this article, we
propose a stratified marginal Cox model to account for secular trend in
cross-sectional SW-CRTs, and derive an explicit expression of the robust
sandwich variance to facilitate power calculations without the need for
computationally intensive simulations. Power formulas based on both the Wald
and robust score tests are developed and compared via simulation, generally
demonstrating superiority of robust score procedures in different finite-sample
scenarios. Finally, we illustrate our methods using a SW-CRT testing the effect
of a new electronic reminder system on time to catheter removal in hospital
settings. We also offer an R Shiny application to facilitate sample size and
power calculations using our proposed methods.
