What is a large effect size?

Cohen's (1988) conventions: d = 0.2 small, 0.5 medium, 0.8 large. In education research, Hattie uses 0.40 as the ‘hinge point.’ In medical research, d = 0.5 is often considered clinically meaningful. Glass's Δ uses only the control group SD instead of pooled SD.

How is Hedges' g different from Cohen's d?

Hedges' g applies a correction factor for small samples (n < 20): g = d × (1 – 3/(4df−1)). For large samples, d ≈ g. Hedges' g is preferred when sample sizes differ significantly between groups or when n < 20.

Cohen's d Effect Size Calculator

Calculate Cohen's d effect size for comparing two group means: d = (M₁–M₂) / pooled SD. Interprets as: small d=0.2, medium d=0.5, large d=0.8. The standard metric for practical significance in behavioural science.

Data set — comma or newline separated

Statistical Results — n = 8

Count (n)8

Sum (Σx)40

Min2

Max9

Range7

Q14

Q35.5

IQR1.5

Mean (μ)5

Median4.5

Mode4

Pop. Variance (σ²)4

Pop. Std Dev (σ)2

Sample Variance (s²)4.571428571

Sample Std Dev (s)2.138089935

Skewness0.65625

Excess Kurtosis-0.21875

How to Use

Enter the values for the scenario you are assessing. Calculate Cohen's d effect size for comparing two group means: d = (M₁–M₂) / pooled SD. Interprets as: small d=0.2, medium d=0.5, large d=0.8. The standard metric for practical significance in behavioural science. Use the cohen's d result to inform your calculation.

Cohen's d Effect Size Calculator

How to Use

Frequently Asked Questions

What is a large effect size?

How is Hedges' g different from Cohen's d?