Determine your confidence interval. The confidence interval represents how accurate your results are. If you have a confidence interval of 5% and the result of your study is 60% positive, the true amount is somewhere between 65% and 55% positive.
Calculate the confidence level with a Z-score chart. The Z-score is related to the standard deviation and is a rule used in statistics. If you want a confidence level of 99%, you will use a different Z-score than if you are seeking a confidence level of 75%.
Calculate the percentage of your sample group that you expect to respond affirmatively in decimal form. This is an educated guess, but it can also be based on mathematical analysis and your preliminary research. For example, if you anticipate 70% of your group to answer positively to a question, use 0.70 as the proportion.
Find your required sample size using this formula, where Z is your Z-score, P is the percentage from Step 3 and I is the confidence interval:
(Z²)(P)(1 - P)/I²