Effect Size Calculator
Calculate Cohen's d, Hedges' g with Step-by-Step Derivation
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Effect Size Formulas
Cohen's d: d = |x̄₁ - x̄₂| / s_pooled
Hedges' g: g = d × (1 - 3/(4(n₁+n₂) - 9))
s_pooled = √(((n₁-1)s₁² + (n₂-1)s₂²) / (n₁+n₂-2))
Cohen: d=0.2 small, 0.5 medium, 0.8 large.
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Hedges' g is preferred for small samples (n < 20) as it corrects bias.
What is Effect Size?
Effect size measures the magnitude of a difference or the strength of a relationship, independent of sample size.
Cohen's d
Standardized mean difference
Hedges' g
Small sample bias correction
Pooled SD
Weighted average of variances
Interpretation
Small, medium, large
💡 Example: x̄₁=10, s₁=2, n₁=30; x̄₂=12, s₂=2.5, n₂=30 → d≈0.87 (large effect)
Applications
Research
Meta-Analysis
Psychology
Education
Medicine
Frequently Asked Questions
What is effect size?▼
Effect size quantifies magnitude of difference or strength of relationship, independent of sample size. Important for practical significance.
What is Cohen's d?▼
Cohen's d = (μ1 - μ2)/σ. Standardized mean difference. d=0.2 small, 0.5 medium, 0.8 large.
What is Hedges' g?▼
Hedges' g adjusts Cohen's d for small sample bias: g = d × (1 - 3/(4(n1+n2) - 9))
How to interpret effect size?▼
Cohen: d=0.2 small, 0.5 medium, 0.8 large. Also consider field, context, practical relevance.
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