Effect Size Calculator
Calculate Cohen's d effect size, which measures the standardized difference between two group means.
Cohen's d
0.5432
Cohen's d vs Group 1 Mean
Formule
## How to Calculate Cohen's d ### Formula **d = (Mean1 - Mean2) / Pooled SD** where Pooled SD = sqrt((s1^2 + s2^2) / 2) Cohen's d expresses the difference between two means in standard deviation units. Guidelines: |d| < 0.2 = negligible, 0.2-0.5 = small, 0.5-0.8 = medium, > 0.8 = large. Effect size is independent of sample size, unlike the p-value.
Exemple Résolu
Group 1: mean = 78, SD = 10. Group 2: mean = 72, SD = 12.
- 01Pooled SD = sqrt((100 + 144) / 2) = sqrt(122) = 11.045
- 02d = (78 - 72) / 11.045 = 6 / 11.045 = 0.5432
- 03|d| = 0.5432 which is a medium effect size
Questions Fréquentes
Why is effect size important?
Statistical significance (p-value) depends on sample size; with enough data, trivial differences become significant. Effect size measures practical significance independently of sample size.
What are typical benchmarks for Cohen's d?
Cohen proposed: 0.2 = small, 0.5 = medium, 0.8 = large. However, context matters. In clinical trials, a small effect may be clinically meaningful; in education, a medium effect might be unremarkable.
Are there other effect size measures?
Yes. Hedges' g corrects for small-sample bias. Glass' delta uses only one group's SD. For correlations, r itself is an effect size. For ANOVA, eta-squared or omega-squared are used.
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