How to Calculate a Sample Size in SPSS
By Stephanie Ellen
Updated September 28, 2017
Sample sizes are needed in statistics because it isn't always possible to conduct an analysis of an entire population. For example, if you want to know the average height of a beech tree, it would be practically impossible to measure every tree. Instead, you take a sample of trees. Deciding how many items to sample is a complicated matter that involves multiple calculations. However, software like IBM's SPSS can help you calculate sample sizes in a snap.
Select the "Data" menu and then click "Select Cases."
Check the "Random sample of cases" radio button, then check the "Filtered" radio button.
Click "Sample" in the center of the dialog box, then check the "Approximately" radio button.
Type a percentage into the box. For example, type "5%" if you want to sample 5 percent of the population.
Click "Continue" and then click "OK." This completes the selection of a random sample.
To check that the system has picked a representative sample, run a descriptive output for the SEX variable. Click "Analyze" and then click "Frequencies." Click "sex" in the left window of the dialog box, then click the arrow key in the middle to transfer "sex" to the right-hand window. Click "Statistics," then place check marks in "Mean," Std. Deviation" and "S.E. mean." Click "Continue" and then click "OK." This will deliver a report that will tell you if you have chosen a representative sample. If you haven't, run through the procedure again with a different percentage.
Stephanie Ellen teaches mathematics and statistics at the university and college level. She coauthored a statistics textbook published by Houghton-Mifflin. She has been writing professionally since 2008. Ellen holds a Bachelor of Science in health science from State University New York, a master's degree in math education from Jacksonville University and a Master of Arts in creative writing from National University.