Bayesian Tests of Two Proportions: A Tutorial With R and JASP

The need for a comparison between two proportions (sometimes called an A/B test) often arises in business, psychology, and the analysis of clinical trial data. Here we discuss two Bayesian A/B tests that allow users to monitor the uncertainty about a difference in two proportions as data accumulate over time. We emphasize the advantage of assigning a dependent prior distribution to the proportions (i.e., assigning a prior to the log odds ratio). This dependent-prior approach has been implemented in the open-source statistical software programs R and JASP. Several examples demonstrate how JASP can be used to apply this Bayesian test and interpret the results.