Sample-size determination for the Bayesian t test and Welch’s test using the approximate adjusted fractional Bayes factor

When two independent means μ(1) and μ(2) are compared, H(0) : μ(1) = μ(2), H(1) : μ(1)≠μ(2), and H(2) : μ(1) > μ(2) are the hypotheses of interest. This paper introduces the R package SSDbain, which can be used to determine the sample size needed to evaluate these hypotheses using the approximate adjusted fractional Bayes factor (AAFBF) implemented in the R package bain. Both the Bayesian t test and the Bayesian Welch’s test are available in this R package. The sample size required will be calculated such that the probability that the Bayes factor is larger than a threshold value is at least η if either the null or alternative hypothesis is true. Using the R package SSDbain and/or the tables provided in this paper, psychological researchers can easily determine the required sample size for their experiments.