Effect Size Calculator

This calculator computes Cohen's d, Hedges' g, and Glass's delta — three related measures of the standardized difference between two group means — from each group's mean, standard deviation, and sample size. Cohen's d divides the mean difference by a Pooled Standard Deviation built from both groups' variability and sample sizes (lines 14-19); Hedges' g then applies a small-sample bias correction, 1 − 3/(4×df − 1) (line 23), that shrinks toward 1 as sample sizes grow. Glass's delta instead divides the mean difference by Group 2's Standard Deviation alone (line 27) and, unlike the other two, is completely unaffected by Group 1's Standard Deviation or either group's sample size.

Group 1 Mean shows a larger measured effect on Cohen's d than Group 2 Mean under a sensitivity check, but this reflects their different default magnitudes (78 vs 72), not an asymmetry in the formula — the true partial derivative with respect to either mean has identical magnitude, 1 / Pooled Standard Deviation (line 11). Neither group's mean has any effect on Pooled Standard Deviation, which is computed purely from the two standard deviations and sample sizes. This calculator does not account for unequal-variance corrections beyond Glass's delta's single-group choice — it offers no Welch-style adjustment, so if the two groups' true variances differ substantially, Cohen's d and Hedges' g's shared pooled-variance assumption may understate or overstate the true standardized difference.