What This Calculator Does
Effect size measures how large a difference between two group means is in practical terms, independent of sample size — unlike a p-value, which only tells you whether a difference is likely to be real.
The Formula: Cohen’s d
This calculator uses Cohen’s d, the most widely used effect size measure for comparing two group means:
- Pooled Standard Deviation = √[((n1−1)×SD1² + (n2−1)×SD2²) / (n1 + n2 − 2)]
- Cohen’s d = |Mean1 − Mean2| / Pooled Standard Deviation
By convention, d ≈ 0.2 is considered a small effect, d ≈ 0.5 a medium effect, and d ≈ 0.8 or higher a large effect, though these thresholds are general guidelines rather than strict rules and can vary by field. This calculator is for general educational and research-planning use, not a substitute for a full statistical analysis.
Last reviewed August 2026