Effect Size Calculator
Effect size measures the practical significance of a result independent of sample size. A statistically significant result can have a trivially small effect size — effect size tells you whether it matters.
- 1Cohen's d = (Mean₁ − Mean₂) / Pooled SD
- 2Small: d = 0.2; Medium: d = 0.5; Large: d = 0.8
- 3r = d / √(d² + 4) converts d to correlation
- 4η² (eta-squared) used for ANOVA effect size
Group A mean 75, Group B mean 68, SD 10=Cohen's d = (75−68)/10 = 0.7 — medium-large effect
| Cohen's d | Size | Practical meaning |
|---|---|---|
| 0.2 | Small | Barely perceptible |
| 0.5 | Medium | Visible to careful observer |
| 0.8 | Large | Obvious practical difference |
| 1.2 | Very large | Major difference |
References
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