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Statistics

Cohen’s d Calculator

Measure the standardized difference between two independent sample means, with an approximate Hedges correction.

Data stays on this deviceConvention explained below
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Check the dataset and sample convention before interpreting results.

See the method

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Use the Calculate button to see your result.

Review the method below for assumptions and conventions.

How to use this tool

  1. Enter sample a, sample b.
  2. Select Calculate to view the result.
  3. Check the method and assumptions below before using the result.

The method, explained

Pooled SD = √[((nA−1)sA²+(nB−1)sB²)/(nA+nB−2)]. d = (mean A−mean B)/pooled SD. Approximate Hedges g = d × [1−3/(4df−1)].

A WORKED EXAMPLE

Using sample a = 8, 10, 9, 12, 11, sample b = 5, 7, 6, 8, 9, the result is 1.8973666 probability. Change these example inputs to match your task; use the method above to check each step.

What to keep in mind

Independent groups and pooled-variance effect-size convention. This is not a paired effect size and does not provide a confidence interval. A descriptive or probability calculation under the stated assumptions. Check the sample design and model before interpreting results.

Reference: NIST: statistical methods handbook

Common questions

Why can Cohen’s d be negative?

The sign follows mean A minus mean B. A negative value means A has the lower sample mean. Reversing the group order reverses the sign while preserving magnitude.

How is Hedges g different?

Hedges g corrects the upward small-sample bias of the standardized mean difference. This page uses the stated common approximation to that correction and labels it as approximate.

Methodology maintained by ClarityKit. How these tools are built and checked.