How to Calculate Cohen's d Effect Size
What is Cohen's d Effect Size?
Cohen's d is a standardised effect size for the difference between two group means, expressed in standard deviation units. d=0.2 is small; d=0.5 medium; d=0.8 large.
Step-by-Step Guide
- 1d = (μ₁ − μ₂) / σ_pooled
- 2σ_pooled = √((σ₁² + σ₂²)/2)
- 3Larger d = more practically significant difference
- 4Allows comparison across studies regardless of measurement scale
Worked Examples
Input
Mean1=50, SD=10 · Mean2=44, SD=10
Result
d = 0.6 (medium effect)
(50-44)/10 = 0.6
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