Calculate Hedges’ g, the small-sample corrected standardised mean difference.
The correction is roughly 5 per cent at a total sample of 20 and under 1 per cent above 100. It is always downward, since Cohen’s d overestimates the population effect in small samples. Meta-analyses use Hedges’ g rather than Cohen’s d precisely because they combine studies with very different sample sizes.
Hedges G
g = d × (1 − 3 ÷ (4N − 9))
g = d × (1 − 3 ÷ (4N − 9)) The correction is roughly 5 per cent at a total sample of 20 and under 1 per cent above 100. It is always downward, since Cohen’s d overestimates the population effect in small samples.
Meta-analyses use Hedges’ g rather than Cohen’s d precisely because they combine studies with very different sample sizes.
This calculator takes 3 inputs: Cohen’s d, Sample size of group 1, Sample size of group 2. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.