📊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.
- 1d = (μ₁ − μ₂) / σ_pooled
- 2σ_pooled = √((σ₁² + σ₂²)/2)
- 3Larger d = more practically significant difference
- 4Allows comparison across studies regardless of measurement scale
Mean1=50, SD=10 · Mean2=44, SD=10=d = 0.6 (medium effect)(50-44)/10 = 0.6
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Fun Fact
Jacob Cohen introduced these benchmarks in 1988, warning they are 'conventional' and context-dependent. A d=0.2 for a life-saving drug is enormous; in education it may be trivial.
References
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