Why do you need to compute a sample size? In research it is very difficlutt to survey an entire population of people. Therefore researchers use a sample size or sample population to measure the attitudes, beliefs, and practices of a given population. A sample size calculator is used to determine how many people you will need to interview or survey in order to collect enough information so that your results are representative of your target population. This technique is used by researchers to ensure that your research have power and is reliable.