Sample Size Calculation and Determination of Length of Time for Clusters
EVENT
We a priori determined the length of the trial period and each step with several considerations in mind. First, we wanted to ensure sufficient sample within each step; however, we also wanted to keep the overall trial period brief enough to minimize the effect of secular trends on the primary outcome. The total volume across all 4 CSCs is about 380 per year and we estimated about 200 of these procedures would meet eligibility criteria based on a review of the previous year’s data. The CSC with the lowest annual volume performs roughly 50 cases per year and we estimated about 30 such cases would meet eligibility criteria based on a review of the previous year’s data. From these considerations, we chose an overall trial period of 14 months, estimating a total sample size of approximately 225. We chose a slightly longer first period to ensure adequate sample size of the pre-AI cohort; a period of 4 months was chosen to ensure that the CSC with the lowest volume would have at least 10 cases during this period, in the event that it was the first hospital to be activated. The subsequent steps were then timed to take place 6 weeks apart, with a period of approximately 3 months at the end of the trial to ensure adequate capture of the post-AI cohort.