Definition

Effect size

A standardized measure of how large a difference or relationship is — Cohen’s d, Pearson’s r, odds ratios — independent of sample size.

Where p-values ask "is there anything here?", effect sizes ask "how much?". Cohen’s d expresses a group difference in standard-deviation units; r expresses relationship strength; odds ratios compare event likelihoods. Because they are standardized, effects can be compared across studies — the foundation of meta-analysis.

Cohen’s rough benchmarks (d = 0.2 small, 0.5 medium, 0.8 large) are a last resort; fields have their own norms, and the honest question is always practical: is a d of 0.3 worth the intervention’s cost? Reviewers increasingly require effect sizes for every reported test.