What this calculator does
Cohen's d (effect size) works out standardised difference between two group means, in pooled standard deviations. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
Formulad = (x̄₁ − x̄₂) / sp; sp = √[((n₁−1)s₁² + (n₂−1)s₂²) / (n₁+n₂−2)]
The inputs explained
| Field | What to enter |
|---|---|
| Group 1 mean | A number. Starts at 85. |
| Group 1 standard deviation | A number. Starts at 12. |
| Group 1 size | A number. Starts at 30. |
| Group 2 mean | A number. Starts at 78. |
| Group 2 standard deviation | A number. Starts at 14. |
| Group 2 size | A number. Starts at 28. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with group 1 mean
Every other input is held at the calculator’s starting values while group 1 mean varies. Select any row to load that scenario into the calculator.