Psychological Methods, Vol 31(1), Feb 2026, 22-39; doi:10.1037/met0000604
Replication studies are essential for assessing the credibility of claims from original studies. A critical aspect of designing replication studies is determining their sample size; a too-small sample size may lead to inconclusive studies whereas a too-large sample size may waste resources that could be allocated better in other studies. Here, we show how Bayesian approaches can be used for tackling this problem. The Bayesian framework allows researchers to combine the original data and external knowledge in a design prior distribution for the underlying parameters. Based on a design prior, predictions about the replication data can be made, and the replication sample size can be chosen to ensure a sufficiently high probability of replication success. Replication success may be defined by Bayesian or non-Bayesian criteria and different criteria may also be combined to meet distinct stakeholders and enable conclusive inferences based on multiple analysis approaches. We investigate sample size determination in the normal–normal hierarchical model where analytical results are available and traditional sample size determination is a special case where the uncertainty on parameter values is not accounted for. We use data from a multisite replication project of social–behavioral experiments to illustrate how Bayesian approaches can help design informative and cost-effective replication studies. Our methods can be used through the R package BayesRepDesign. (PsycInfo Database Record (c) 2026 APA, all rights reserved)