# Statistical Evidence in Experimental Psychology: An Empirical Comparison Using 855 t Tests

##### Wetzels, R.; Matzke, D.; Lee, M.D.; Rouder, J.N.; Iverson, G.J.; Wagenmakers, E.-J.

#### Perspectives on Psychological Science: a Journal of the Association for Psychological Science 6(3): 291-298

#### 2011

**ISSN/ISBN: 1745-6916**

PMID: 26168519

DOI: 10.1177/1745691611406923

Accession: 036096798

Statistical inference in psychology has traditionally relied heavily on p-value significance testing. This approach to drawing conclusions from data, however, has been widely criticized, and two types of remedies have been advocated. The first proposal is to supplement p values with complementary measures of evidence, such as effect sizes. The second is to replace inference with Bayesian measures of evidence, such as the Bayes factor. The authors provide a practical comparison of p values, effect sizes, and default Bayes factors as measures of statistical evidence, using 855 recently published t tests in psychology. The comparison yields two main results. First, although p values and default Bayes factors almost always agree about what hypothesis is better supported by the data, the measures often disagree about the strength of this support; for 70% of the data sets for which the p value falls between .01 and .05, the default Bayes factor indicates that the evidence is only anecdotal. Second, effect sizes can provide additional evidence to p values and default Bayes factors. The authors conclude that the Bayesian approach is comparatively prudent, preventing researchers from overestimating the evidence in favor of an effect.