The Publication Manual of the American Psychological Association (American Psychological Association,
2001, 2010) calls for the reporting of effect sizes and their confidence intervals. Estimates of effect
size are useful for determining the practical or theoretical importance of an effect, the relative contributions
of factors, and the power of an analysis. We surveyed articles published in 2009 and 2010 in the
Journal of Experimental Psychology: General, noting the statistical analyses reported and the associated
reporting of effect size estimates. Effect sizes were reported for fewer than half of the analyses; no article
reported a confidence interval for an effect size. The most often reported analysis was analysis of
variance, and almost half of these reports were not accompanied by effect sizes. Partial
2 was the most
commonly reported effect size estimate for analysis of variance. For t tests, 2/3 of the articles did not
report an associated effect size estimate; Cohen’s d was the most often reported. We provide a
straightforward guide to understanding, selecting, calculating, and interpreting effect sizes for many types
of data and to methods for calculating effect size confidence intervals and power analysis.
Keywords: effect size, eta squared, confidence intervals, statistical reporting, statistical interpretation