Hedges’ g Calculator
Standardize an independent-group mean difference using a pooled SD and approximate small-sample correction.
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Compute a pooled-SD standardized difference for two independent groups. The direction is Group A minus Group B. Defaults are fictional. Enter standard deviations, not standard errors.
Calculation results
Hedges’ g ≈ -0.495177
Pooled SD: 4. Cohen’s d: -0.5. Approximate correction J: 0.990354.
A negative value means Group A has the lower mean. Whether that is beneficial depends on the outcome scale.
Method and limits
Pooled SD = √[((nA−1) SDA² + (nB−1) SDB²)/(nA+nB−2)]. d = (meanA−meanB)/pooled SD. With df = nA+nB−2, J ≈ 1−3/(4df−1) and g = Jd. This is an approximate small-sample correction, not the exact gamma-function correction. No confidence interval is computed.
Use for independent groups measuring the same underlying construct. This calculation is not for paired changes, clustered data, medians, or incompatible outcomes. Pooling requires further assumptions, including whether SD differences reflect measurement scale rather than population variability. Do not apply universal clinical-importance labels to g.
Cochrane: effect measures and calculations · Read the practical guide · Download blank worksheet (TXT)