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

##### Fu, Q.; Hoijtink, H.; Moerbeek, M.

#### Behavior Research Methods 53(1): 139-152

#### 2021

**ISSN/ISBN: 1554-3528**

PMID: 32632740

DOI: 10.3758/s13428-020-01408-1

Accession: 078924432

When two independent means μ1 and μ2 are compared, H0 : μ1 = μ2, H1 : μ1≠μ2, and H2 : μ1 > μ2 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.