Comprehensive Calculator Guide
This calculator gives Cohen’s d, the difference between two group means in standard-deviation units, showing whether it is small, medium or large regardless of sample size. A new teaching method averaging 78 against 72, with SDs of 10 and 12, has d = 0.54, a medium effect.
Effect size
d = (mean₁ − mean₂) ÷ pooled SD. Cohen’s guides: 0.2 small, 0.5 medium, 0.8 large, but practical importance depends on the field. Significance says a difference is real; effect size says it matters.
How to Use
- Enter group 1’s mean, SD and size.
- Enter group 2’s.
- Press “Calculate” to see d and its size.
Formula Used
d = (x̄₁ − x̄₂) ÷ s_pooled
Practical Examples
- Example 1
- Teaching: 0.54.
- Example 2
- Exercise: 1.45.
Important Tips
- Report d with p and a CI.
- Interpret in context.
- Use Hedges’ g for tiny samples.
Common Mistakes to Avoid
- Using one group’s SD.
- Dismissing small effects.
- Confusing d with significance.
Frequently Asked Questions
- What is d?
- A standardised mean difference.
- Large?
- 0.8+.
- Negative?
- Yes; read its size.