Confidence interval for a mean or a proportion
The mean interval uses n−1 degrees of freedom. The proportion interval is the Wilson score interval.
Questions
Which confidence interval does this calculator use?
For a mean it uses the Student t interval from the sample size, mean, and standard deviation. For a proportion it uses the Wilson score interval, which behaves better than the plain normal interval when the proportion is near 0 or 1.
What confidence level should I pick?
95% is the usual reporting level. The box accepts another level, such as 0.90 or 0.99, as a proportion rather than a percent.
Is this a confidence interval for a difference?
No. This page is one mean or one proportion. A difference between two groups needs the two-sample formula and is not calculated here.
Educational only. Not medical advice, not a regulatory determination, and not a substitute for qualified RA/QA, legal, clinical, or statistical counsel. Confidence interval calculator does not submit to FDA, Notified Bodies, or IRBs.