# Thread: Calculating 95% Confidence Intervals for Prevalence (%)

1. ## Calculating 95% Confidence Intervals for Prevalence (%)

Hi All,

I am hoping to get some advice on how I can calculate the 95% Confidence Intervals for prevalence percentages. My sample size is 180, and I am currently using the statistical package SPSS.

If anyone has any suggestions they would be appreciated!

Thanks.

2. ## Re: Calculating 95% Confidence Intervals for Prevalence (%)

A standard formula for the CI of a proportion is 1.96 x sqrt(p(1-p)/n)
https://onlinecourses.science.psu.edu/stat200/node/48

3. ## The Following User Says Thank You to katxt For This Useful Post:

bozatron (01-10-2017)

4. ## Re: Calculating 95% Confidence Intervals for Prevalence (%)

Thanks so much for the reply. Will give that a go!

Does any one know if there is a way to calculate 85% CIs for percentages using SPSS?

Thanks!

5. ## Re: Calculating 95% Confidence Intervals for Prevalence (%)

But would the assumption here be np>5 and n(1-p)>5?

What is the CI calculation procedure if that assumption is violated?

6. ## Re: Calculating 95% Confidence Intervals for Prevalence (%)

I used np>10 and n(1-p)>10 as my assumptions. If this is violated I have used the Wilson estimate method (also known as the plus four estimate). Hope that's the correct way of doing it so please let me know if it's not!

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