Chapter 14: ANOVA: Effect Sizes, Simulating Interaction Versus Main Effects, and a Modified ANOVA Table

The fifth edition of the American Psychological Association’s (APA, 2001)Publication Manual contains a number of changes to the standard ANOVA table. Among the changes are inducements to report obtained p values and include η, a measure of effect size.

The revision to the APA style manual was influenced considerably by Thompson (1996; see Thompson & Vacha-Haase, 2000). He recommended that effect sizes, “can and should be reported and interpreted in all studies, regardless of whether statistical tests are reported. AERA [the American Educational Research Association] should venture beyond the APA, and require” (Thompson, 1996, p. 29) reporting of effect sizes.