Effect Size Calculator – Equal Groups
Find Cohen’s d and its r conversion for two independent groups of equal size.
Formula
s = √((s₁²+s₂²)/2); d = (m₁−m₂)/s; r = d/√(d²+4); r² = d²/(d²+4).
Cohen’s d ≈ ((12)-(10))÷(2) · Converted r ≈ (1)÷sqrt((1)^2+4) · Squared r ≈ (1)^2÷((1)^2+4)
How to use
- Enter each group mean.
- Enter sample standard deviations in the same unit.
- Read signed d, r and r².
Examples
- Means 12 and 10 with both SDs 2 give d=1.
Frequently asked questions
Why equal sample sizes?
The unweighted pooled variance and this d-to-r conversion assume equal group sizes.
What does a negative d mean?
Group 1 has the lower mean. Reversing groups reverses the sign.
Is r² a causal percentage?
No. It is the squared converted correlation; it does not establish causation or significance.
Sources
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