Cohen's d Calculator
Compare two groups by calculating Cohen's d effect size with pooled standard deviation, interpretation guidance, and supporting statistics.
Group 1
Group 2
Cohen's d
0.719
Pooled SD (sp)
10.0091
Interpretation
Medium effect size.
How to Use This Calculator
- Enter the mean, standard deviation, and sample size for each group.
- Ensure sample sizes are at least 2 to compute a pooled standard deviation.
- Review the resulting Cohen's d and pooled standard deviation.
- Use the interpretation to gauge the magnitude of the effect.
Formula
sp = √[((n₁ − 1)·SD₁² + (n₂ − 1)·SD₂²) / (n₁ + n₂ − 2)]
Cohen's d = (M₁ − M₂) / sp
Cohen's benchmarks: 0.2 (small), 0.5 (medium), 0.8 (large) — though practical interpretation depends on context.
Full Description
Cohen's d standardizes mean differences, allowing comparison of effect sizes across studies or metrics. It is commonly reported in psychology, education, and medical research to contextualize treatment effects or group differences.
Because it uses pooled standard deviation, Cohen's d assumes similar variance between groups. When variances differ substantially, consider Hedge's g or Glass's Δ variants.
Frequently Asked Questions
Can I use unequal sample sizes?
Yes. The pooled standard deviation accounts for sample sizes n₁ and n₂.
What if the pooled SD is zero?
If both groups have zero variance, the effect size is undefined; we report zero for stability.
How does Cohen's d relate to t-tests?
It is a standardized effect derived from group means; t-statistics can be converted to Cohen's d when needed.
Should I report confidence intervals?
Best practice includes CI for Cohen's d, but this calculator focuses on the point estimate for simplicity.