Cohen's D Calculator
Calculate Cohen's d to measure the practical significance of the difference between two group means. Also computes Hedges' g (bias-corrected), distribution overlap, and the common language effect size.
Cohen's d is the most widely used measure of effect size for comparing two groups. It expresses the difference between group means in standard deviation units.
The Formula:
d = (M1 - M2) / s_pooled
where s_pooled = sqrt[((n1-1)s1^2 + (n2-1)s2^2) / (n1+n2-2)]
Cohen's Benchmarks:
| |d| Value | Interpretation | |-----------|----------------| | < 0.2 | Negligible | | 0.2 | Small | | 0.5 | Medium | | 0.8 | Large |
Hedges' g corrects Cohen's d for small-sample upward bias using the factor J = 1 - 3/(4*df - 1). For samples of 20+ per group, the difference is small.
Common Language Effect Size (CLES): This translates d into a probability. If d = 0.5, there is about a 64% chance that a randomly selected person from Group 1 scores higher than a randomly selected person from Group 2.
Distribution Overlap: Shows how much the two bell curves overlap. Even a "large" effect (d = 0.8) still has about 69% overlap, which is why individual predictions from group differences are unreliable.