Sample size estimation for the ratio of count outcomes in a cluster randomized trial

Music City Center 

Count outcomes frequently occur in cluster randomized trials, where the ratio of count outcomes is often used to assess the effectiveness of an intervention. However, in practice, cluster sizes typically vary across clusters, and sample size estimation based on a constant cluster size assumption may lead to Underpowered studies. To address this issue, we propose using the generalized estimating equation (GEE) method to test the ratio of two count outcomes and introduce closed-form formulas for sample size and power calculation. In particular, the ratio of count outcomes has been frequently used to evaluate vaccine efficacy in cluster randomized trials, where the aim is to demonstrate that the vaccine reduces the incidence rate of a disease compared to placebo or active control. To illustrate the application of the proposed method in a vaccine efficacy cluster randomized trial, we present sample size calculation for the ratio of two count outcomes, accounting for randomly varying cluster sizes.

vaccine efficacy

cluster randomized trial

sample size 

ENAR