A Practical Guide to Power Analyses of Moderation Effects in Multisite Individual and Cluster Randomized Trials

Abstract

Multisite trials that randomize individuals (e.g., students) within sites (e.g., schools) or clusters (e.g., teachers/classrooms) within sites (e.g., schools) are commonly used for program evaluation because they provide opportunities to learn about treatment effects as well as their heterogeneity across sites and subgroups (defined by moderating variables). Despite the rich opportunities they present, a critical step in ensuring those opportunities is identifying the sample size that provides sufficient power to detect the desired effects if they exist. Although a strong literature base for conducting power analyses for the moderator effects in multisite trials already exists, software for power analysis of moderator effects is not readily available in an accessible platform. The purpose of this tutorial paper is to provide practical guidance on implementing power analyses of moderator effects in multisite individual and cluster randomized trials. We conceptually motivate, describe, and demonstrate the calculation of statistical power and minimum detectable effect size difference (MDESD) using highly accessible software. We conclude by outlining guidelines on power analysis of moderator effects in multisite individual randomized trials (MIRTs) and multisite cluster randomized trials (MCRTs).