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Work out how many people you need for a survey (with margin of error and population size), to estimate a mean, or to compare two groups with a given power.
Also covers: Survey sample size calculator ยท Power analysis calculator ยท Margin of error sample size
50% if unsure.
Sample size needed
385
For ยฑ5% at 95% confidence.
Working
Use 50% when you don't know the proportion: it gives the largest, safest sample size.
A survey estimating a percentage, an estimate of a mean, or a study comparing two groups.
For estimates, the confidence level and margin of error. For comparisons, the effect size you want to detect, ฮฑ, and power.
Always rounded up, with the formula and working. Plan for extra participants to cover non-response and drop-outs.
At 95% confidence, a ยฑ5% margin of error, and an expected proportion of 50%, n = 1.96ยฒ ร 0.25 / 0.05ยฒ = 384.2, rounded up to 385. It barely changes for any large population.
Only when the sample is a sizeable fraction of the population. Enter the population size and the finite-population correction lowers the number needed, e.g. to 278 for a population of 1,000.
The probability of detecting an effect that really exists. 80% is the usual minimum; higher power or a smaller effect size needs more participants.
No. Every calculation runs in your browser, and nothing you enter is sent to a server.
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