How to Get Geometric Means in SPSS
By Warren Davies
SPSS is a statistical analysis software package that allows a wide range of manipulations and procedures to be performed on datasets. Unfortunately, one procedure that is notably absent is the geometric mean. This is a central tendency statistic used when a dataset is skewed or has been altered by some relative function. Despite being missing from SPSS, the geometric mean can be calculated using a combination of SPSS’s “Compute Variable” function, and a scientific calculator found on most computer operating systems for free.
Load up SPSS. On the welcome screen, select “Open an existing data source” and then double-click “More Files.” Locate your data file on your computer, click it once and then click “Open.”
Click “Transform” from the menu bar at the top of the screen, then click “Compute.” In the “Target Variable” box, type a name for the new variable. This will be the log of the variable that you want the geometric mean of, so “log_yourvariable” would be a suitable descriptive name. In the “Numeric Expression” box, type “lg10(yourvariable), replacing “yourvariable” with the actual name of the variable you want to work with.
Get the arithmetic mean for your new variable by clicking “Analyze,” then “Descriptive Statistics” and then “Descriptives.” Drag “log_yourvariable” from the left box into the right box and click "OK." Note that if this variable is split into groups through a separate grouping variable, you must split the file before you do this. Go to “Data,” click “Split File” and then click “Compare Groups.” Move your grouping variable into the “Groups Based On” box. Select “OK” to split the file. Simply return to this window and click “Analyze all cases, do not compare groups” when you no longer need the split file.
Load up the scientific calculator on your computer. To get the geometric mean, you must calculate 10 to the power of the arithmetic mean of your log transformed variable. In your “Output” window, scroll down until you find your variable, then locate the “Mean” column. In the calculator window, type “10” and then press the “x^y” -- sometimes labeled “y^x" -- button. Type in the mean from the table. Press the “Equals” button. The result is the geometric mean.
Repeat this process on any other variables for which you need the geometric mean.
Warren Davies has been writing since 2007, focusing on bespoke projects for online clients such as PsyT and The Institute of Coaching. This has been alongside work in research, web design and blogging. A Linux user and gamer, warren trains in martial arts as a hobby. He has a Bachelor of Science and Master of Science in psychology, and further qualifications in statistics and business studies.