How this calculator works
The denominator pools within-group sample variances with n - 1 weights. This independent-groups definition assumes a common population variance. The additional Hedges estimate applies the approximate small-sample multiplier 1 - 3/(4df - 1); it is labelled approximate because an exact gamma-function correction differs slightly. Each group needs at least two observations. Raw groups may have different sizes.
Formula
sp² = ((n1 - 1)s1² + (n2 - 1)s2²)/(n1 + n2 - 2); d = (mean1 - mean2)/sp; g ≈ (1 - 3/(4df - 1))d.
Worked example
Means 12 and 10, standard deviations 2 and 2, and sample sizes 10 and 10 give sp = 2 and d = 1. The approximate corrected g is 0.957746.
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Frequently asked questions
Can I use paired observations?
This page uses an independent-groups denominator. For paired designs the standard deviation of differences or another justified denominator is needed.
Is d a significance test?
No. It describes a standardised mean difference. It supplies neither a p-value nor proof of a practical effect.
What if both groups are constant?
A zero pooled standard deviation makes the standardised difference undefined, even if the means match.