What are the samplesize requirements for estimating a differ

What are the sample-size requirements for estimating a difference of proportions? What should you do if your sample does not meet those criteria?

Solution

Sample Proportions

The sample proportions are what you expect the results to be. This can often be determined by using the results from a previous survey, or by running a small pilot study. If you are unsure, use proportions near to 50%, which is conservative and gives the largest sample size. Note that this sample size calculation uses the Normal approximation to the Binomial distribution. If, one or both of the sample proportions are close to 0 or 1 then this approximation is not valid and you need to consider an alternative sample size calculation method.

Sample size

This is the minimum sample size for each group to detect whether the stated difference exists between the two proportions (with the required confidence level and power). Note that if some people choose not to respond they cannot be included in your sample and so if non-response is a possibility your sample size will have to be increased accordingly. In general, the higher the response rate the better the estimate, as non-response will often lead to biases in you estimate.