How to use this calculator
Choose precision, planned power, or achieved power and enter assumptions before data collection.
Formula and method
Mean margin n≈(zσ/E)²; proportion margin n≈z²p(1−p)/E². Two-group methods use normal approximations.
Worked example
95% confidence, p=.5 and E=.05 gives ceil(1.96²×.25/.05²)=385 people.
Assumptions and supported scope
Two-sided independent equal-allocation designs. Planning power uses normal approximations, not exact binomial or noncentral t power. No cluster/design effect, finite population, one-sided or sequential design. Round upward and specify attrition.
Method references
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