Confidence Interval for a Proportion Calculator (Free)
Use this free confidence interval calculator for a proportion to estimate a population proportion. Enter the successes, sample size, and confidence level to get p̂, the standard error, z*, the margin of error, and the interval.
The formula
The interval is p̂ ± z*·√[ p̂(1 − p̂)/n ], where p̂ = x/n and z* is the critical value for your confidence level.
Original price was: $109.99.$54.99Current price is: $54.99.
How to use it
- Enter successes x, sample size n, and the confidence level.
- Press Compute.
Related: the Confidence Interval Calculator (for a mean) and more free math calculators.
Frequently asked questions
What does the interval mean?
With the stated confidence (e.g., 95%), the method captures the true proportion in such an interval that often over repeated samples.
Original price was: $109.99.$54.99Current price is: $54.99.
Which method is used?
The normal (Wald) interval, p̂ ± z*·SE.
Original price was: $109.99.$54.99Current price is: $54.99.
How is z* found?
From the confidence level using the inverse normal distribution (e.g., 1.96 for 95%).
Original price was: $27.99.$17.99Current price is: $17.99.
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