Effect Size Calculator — Formula
## 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.
### 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.
Esempio Risolto
Group 1: mean = 78, SD = 10. Group 2: mean = 72, SD = 12.
- Pooled SD = sqrt((100 + 144) / 2) = sqrt(122) = 11.045
- d = (78 - 72) / 11.045 = 6 / 11.045 = 0.5432
- |d| = 0.5432 which is a medium effect size