Effect Size Calculator (Cohen's d)
Calculate Cohen's d and Hedges' g effect sizes to measure the practical significance of the difference between two group means. While p-values tell you if a difference exists, effect size tells you how large it is.
Effect size quantifies the magnitude of the difference between two groups.
Cohen's d: d = (M₁ - M₂) / s_pooled
Where s_pooled = √[((n₁-1)s₁² + (n₂-1)s₂²) / (n₁+n₂-2)]
Cohen's Benchmarks
- Small: d = 0.2
- Medium: d = 0.5
- Large: d = 0.8
Hedges' g applies a correction for small sample bias.
Why effect size matters: A study with a large sample may find a statistically significant but practically trivial difference. Effect size tells you whether the difference matters in practice.