How to Calculate a Sample Size Population

By Ethan Schwartz

The sample size of a study refers to the number of data points collected. A well-designed study with an adequate sample size will usually have some predictive power, because the researchers collected enough data points to make reasonable assumptions about the target population based on their sample. However, a study with an insufficient sample size can easily come to the wrong conclusions. Scientists and pollsters can calculate the sample size that they need to survey by using a calculator that takes the accuracy needed to make correct predictions into account.

Choose the confidence interval (also known as the margin of error) that you want your study to have. Statisticians express a confidence interval as the plus/minus percentage; for instance, if your study needs to be within a five-point margin, you would choose a confidence interval of plus or minus 2.5 percent. You can choose a larger confidence interval if the results of your study need to be less exact.

Select the confidence level (also known as the risk level) that you want your study to have. A confidence level describes the probability that your sample will accurately describe the total population. For instance, if you choose a 95 percent confidence level, 95 percent of your sample will represent the total population. For a more precise and accurate poll or study, you can use a 99 percent confidence interval.

Find your total population size. This depends on the region you wish to survey and the type of poll you have designed. For example, if you wanted to perform a political poll to predict the results of the next U.S. presidential election, your population would be the total number of eligible or likely voters in the U.S. You will probably have to do research in order to find your total population size.

Access the sample size calculator located in the Resources section. This calculator will tell you your necessary sample size, based on the parameters that you give it.

Select your chosen confidence level by filling in the respective field next to the 95 percent or 99 percent button.

Enter your chosen confidence interval in the text box on the calculator.

Fill in the blank for your total population size, and then click the “Calculate” button. This calculator will give you the sample size necessary to fit the parameters that you have outlined.

Adjust your parameters as necessary. While calculating the necessary sample size is important, you also need to take the cost per sample into account. If the calculator returns a sample size that is far beyond your budget, you will have to either decrease your confidence level or your confidence interval in order to accommodate your budget.

About the Author

Ethan Schwartz has been a scientific and freelance writer for a year, writing scientific literature for "Gene Therapy" and articles on education, health and personal finance for eHow. Schwartz graduated in 2009 with a Bachelor of Science in biological sciences and will begin medical school in July 2010.