Effect Size Calculator

How this is calculated

Cohen's d is the difference between two group means divided by their pooled standard deviation. Hedges' g applies a small-sample correction to Cohen's d and is generally preferable when either group has fewer than about 20 participants. Rough interpretation benchmarks: |d| < 0.2 negligible, 0.2–0.5 small, 0.5–0.8 medium, > 0.8 large — though these vary by field and shouldn't be applied rigidly.

Odds ratio and risk ratio are calculated from a 2×2 event table using standard formulas, with 95% confidence intervals from the log-scale standard error. If any cell in your table is zero, standard formulas are undefined — a continuity correction (commonly adding 0.5 to each cell) is needed, which most dedicated meta-analysis software applies automatically.

For the full formulas and when to use each measure, see our guide to calculating effect size.