Although we normally start with the confidence level, C, we want for our confidence interval, sometimes you need to work backwards from the margin of error, E, and find the confidence level,

In this problem, you are given some summary data about a survey and asked you to find the confidence level and the confidence interval for the population proportion if the margin of error, E, is also given.

Although you can do this problem using StatCrunch, I find it to be one of those problems where Excel is quicker and easier, particularly if you use my “calculator.” Enter your summary data in the blue cells. Find the answers in the yellow cells below. This approach only works if the sample size and p are large enough that you can use the normal approximation to the binomial.

You may also download a copy of this workbook.

Download a copy Confidence Level for Proportion CI V1.02