About this calculator
Planning a study
Before collecting data, you can decide how precise you want your estimate to be (the margin of error E) and solve for the sample size n that achieves it.
- For a mean you need a value for σ, from a previous study, a pilot sample, or a rough guess such as range ÷ 4.
- For a proportion with no prior estimate, use p̂ = 0.5. It gives the largest (safest) sample size.
Always round the answer up to the next whole number. Cutting the margin of error in half requires about four times as many people.