Cohen's h Calculator
Measure the effect size for the difference between two proportions. Enter both proportions as percentages.
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What Cohen's h is for
Cohen's h is the effect size for the difference between two proportions, such as conversion rates or the share choosing an answer in two groups. It uses an arcsine transformation so that a given gap counts as a larger effect near 0% or 100% than it does in the middle, where proportions have more room to move by chance.
Reading the size
By Cohen’s conventions, 0.2 is a small effect, 0.5 is medium and 0.8 is large. Unlike a raw difference in percentage points, h is comparable across studies and is what A/B test power and sample size calculations are usually based on.
Frequently asked questions
How is this different from just subtracting the percentages?
A 5-point gap means something different at the extremes than in the middle. Cohen’s h adjusts for that so effects are comparable regardless of where the proportions sit.
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