We want to conduct a multi-center randomized clinical trial with two arms (2 treatments : a superiority trial) stratified on three centers (three sites).
The primary outcome variable is a continuous variable (presented as mean, sd).
How can i estimate the sample size needed for this stratified trials (for each center)?
I found many packages in R that deal with sample size estimation, but i do not I did not find any that is suitable for this analysis.
Thank you
Best Answer
If you are assuming the treatment effect size and the sd of the outcome are the same in all three sites, then you don't need to account for center in your power calculation.
This doesn't mean that the stratified randomization isn't likely to be improving your power in actual practice. Stratified randomization, matched randomization, minimization, and other forms of restricted randomization all increase your power through balancing (among the treatment arms) the distribution of covariates that are predictive of the outcome. However in your case, you are already accounting for this benefit if you are assuming a common sd for the outcome within each site. The sensibility of that assumed sd is what will largely drive the sensibility of your power calculation.