Skip to content
ENExplore Calculators

Effect Size For Study Calculator

Work out effect size for study instantly with clear inputs, formula shown and shareable results.

About Effect Size For Study Calculator

Cohen's d standardises a mean difference by the pooled standard deviation. Means of 24.6 and 21.2 with standard deviations of 5.4 and 5.9 give a pooled SD of 5.66 and d = 0.60 - a medium effect, with the two distributions still overlapping about 76%. Cohen's 0.2, 0.5 and 0.8 benchmarks are conventions, not laws.

Formulas

Cohen's d

d = (mean1 - mean2) / pooled standard deviation

Pooled standard deviation

sqrt((sd1^2 + sd2^2) / 2) for equal group sizes

Worked example

With the default values below, Effect Size For Study Calculator returns Cohen's d of 0.6012. Change any input above and every figure here is recalculated in your browser.

Inputs used

Mean of the first group
24.6
Mean of the second group
21.2
Standard deviation of the first groupminimum 0
5.4
Standard deviation of the second groupminimum 0
5.9

Results

Cohen's d
0.6012
Pooled Standard Deviation
5.656
Distribution Overlap
76.4%
Effect Magnitude
Medium effect

Step by step

  1. 1Values used
  2. 2Cohen's d
  3. 3Pooled standard deviation
  4. 4Cohen's d= 0.6012
  5. 5Pooled Standard Deviation= 5.656
  6. 6Distribution Overlap= 76.4
  7. 7Effect Magnitude= Medium effect

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why report effect size as well as p values?

Because a p value only says whether an effect exists, not how large it is. With a big sample a trivial difference can be highly significant.

Are Cohen's benchmarks universal?

No. Cohen himself called them arbitrary. In some fields d = 0.2 is a major finding; in others 0.8 is unremarkable.

Related Calculators

Effect Size For Study Calculator – Calculate Cohen's d