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Sample size calculator
Enter the effect size you need to detect. You get the required sample per group, targets adjusted for attrition, and the justification sentence to paste into your protocol — the part committees actually query.
Reviewers rarely dispute the arithmetic. They dispute an assumed effect size with no source, chosen because it produced a convenient sample. It needs referencing to comparable work, or justifying as the smallest effect worth detecting.
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This calculator was checked by Sidra Khan, Lead Biostatistician
In short
How do you calculate sample size?
Sample size for comparing two groups is determined by the effect size you want to detect, the significance level, and the statistical power you require. Conventionally 80% power at α = .05 is used. Detecting a medium effect (d = 0.5) under those conditions requires approximately 64 participants per group.
Further reading
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