Keywords: Stepped wedge; Cluster randomized trials; Health service research; Sample size; Generalized estimating equations (GEE), Small sample correction
In stepped wedge cluster randomized trials, intact clusters of individuals switch from control to intervention from a randomly-assigned period onwards. Such trials are becoming increasingly popular in health service research. When a closed cohort is recruited from each cluster for longitudinal follow-up, proper sample size calculation should account for three types of intraclass correlations: the within-period, the inter-period and the within-individual correlations. Setting the latter two correlation parameters to be equal accommodates cross-sectional designs. We propose sample size procedures for continuous and binary responses within the framework of generalized estimating equations that employ a block exchangeable within-cluster structure defined from the correlation types. For continuous responses, we show the intraclass correlations affect the power only through two eigenvalues of the correlation matrix. We demonstrate that analytical power estimates agree well with the simulated power even for as few as 10 clusters, when data are analyzed using bias-corrected estimating equations for the correlation parameters concurrently with a bias-corrected sandwich variance estimator.