Sample Size Calculator

What does a sample size calculator do? When you know how accurate you want your data to be, and you know the size of your target population, then you need to calculate how big a sample you need to draw from that population in order to have survey results that can be projected onto the target population. See below for a basic explanation of the relevant statistic procedures, and see the box at right to choose a sample size calculator.

About the Sample Size Calculator

Sometimes you know how accurate you want your data to be, i.e., you have a desired margin of error (see Basic Statistical Testing for how to calculate the margin of error), and you know the size of the population you want to get information on. Then you need to know how many people in that population you need to actually talk to in order to have results that can be projected onto the entire population with the desired amount of accuracy.

To calculate the desired sample size when your results are going to be in proportions or percentages, you’ll need to know your desired confidence level, desired margin of error, and, if known, the estimated proportion or percentage you expect to find. If you don’t know, then you can guess 50 percent, since a 50-50 split is the worse case scenario. 

To calculate the desired sample size when your results are in means or averages, you don’t need to give an estimated result, but you will need to estimate a standard deviation for your data set.