Sample Size: Rare Event p = 0.1, 95% Confidence, ±5% MOE
For a rare outcome with expected proportion p = 0.1, 95% confidence, and ±5% margin of error, you need 139 participants.
How to use this tool
- Enter expected proportion (p), z* (confidence level) and margin of error (e) in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your required sample size (n) and the full breakdown beneath it.
Calculate the required sample size when the expected proportion of the outcome is low (p = 0.1), which reduces the required sample compared to p = 0.5.
Frequently asked questions
- Why use p = 0.5 when the proportion is unknown?
- p × (1−p) is maximised at p = 0.5, giving the most conservative (largest) required sample size. If you have a prior estimate of p, use it to get a smaller required n.