Effect size calculator
Calculate Cohen's d (and Hedges' g for two groups) from summary statistics. Runs in your browser.
Bands use |d|: below 0.2 very small, 0.2 to under 0.5 small, 0.5 to under 0.8 medium, 0.8 or more large. These are conventions, not a clinical cutoff.
Questions
What is Cohen's d?
Cohen's d is a standardized mean difference: the difference between two means divided by a standard deviation. It expresses the effect in SD units so studies on different scales can be compared.
What is the difference between Cohen's d and Hedges' g?
Hedges' g multiplies d by a small-sample correction, 1 − 3 / (4(n1+n2−2) − 1). Cohen's d slightly overestimates the population effect in small samples; g reduces that bias. With large samples they are nearly equal.
Why are the small, medium and large bands not a clinical rule?
Cohen proposed 0.2, 0.5 and 0.8 as rough conventions for when no better context exists. Whether an effect matters clinically depends on the outcome, the minimal clinically important difference, risks and costs — not on these bands.
Educational only. Not medical advice, not a regulatory determination, and not a substitute for qualified RA/QA, legal, clinical, or statistical counsel. Effect size calculator does not submit to FDA, Notified Bodies, or IRBs. This is research arithmetic, not a diagnosis.