Cohen's d Calculator (Effect Size)
Enter each group’s mean, standard deviation and size.
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Cohen's d
Cohen's d
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Hedges' g (small-sample corrected)
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Pooled standard deviation
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Interpretation
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Probability a random member of group 1 scores higher
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Cohen's d Calculator (Effect Size) by TheCalcTools
How the Cohen's d Calculator (Effect Size) works
Cohen’s d is the difference in means divided by the pooled standard deviation, so it measures the size of an effect in standard-deviation units, independent of sample size.
Cohen’s rough benchmarks are 0.2 small, 0.5 medium and 0.8 large. Hedges’ g removes a small upward bias in small samples.
The formula
d = (x̄₁ − x̄₂) ÷ s_pooled; g = d × (1 − 3 ÷ (4(n₁ + n₂) − 9))
Worked example
Using the default values shown in the calculator:
| Group 1 mean | 105 |
|---|---|
| Group 1 standard deviation | 15 |
| Group 1 size | 40 |
| Group 2 mean | 98 |
| Group 2 standard deviation | 14 |
| Group 2 size | 42 |
- Means 105 and 98 with SDs 15 and 14: d = 0.4829 (Small).
- Hedges’ g = 0.4783.
Result:
| Cohen's d | 0.4829 |
|---|---|
| Hedges' g (small-sample corrected) | 0.4783 |
| Pooled standard deviation | 14.4961 |
| Interpretation | Small |
| Probability a random member of group 1 scores higher | 63.4% |
Frequently asked questions
Why report effect size as well as p-values?
A tiny effect can be ‘significant’ in a huge sample. Effect size shows whether the difference matters.
Sources and further reading
- Wikipedia: Effect size
- Cohen J (1988). Statistical Power Analysis for the Behavioral Sciences, 2nd ed.
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