ANOVA with Repeated Measures using SPSS

Overview

An ANOVA with repeated measures is for comparing three or more group means where the participants are the same in each group. This usually occurs in two situations - when participants are measured multiple times to see changes to an intervention or when participants are subjected to more than one condition/trial and the response to each of these conditions wants to be compared. For a complete guide on ANOVA with Repeated Measures, please go to our guide here.

Example

Heart disease is one of the largest causes of premature death and it is now known that chronic, low-level inflammation is a cause of heart disease. Exercise is known to have many benefits including protection against heart disease. A researcher wished to know whether this protection against heart disease might be afforded by exercise reducing inflammation. The researcher was also curious as to whether this protection might be gained over a short period of time or whether it took longer. In order to investigate this idea the researcher recruited 20 participants who underwent a 6-month exercise training program. In order to determine whether inflammation had been reduced, he measured the inflammatory marker called CRP pre-training, 2 weeks into training and post-6-months-training.

Assumptions

• The dependent variable is interval or ratio (continuous). (see our guide on Types of Variable)
• Dependent variable is approximately normally distributed. (see SPSS article on Testing for Normality)
• Sphericity (see our guide on Sphericity).
• One independent variable where participants are tested on the same dependent variable at least 2 times.

Test Procedure in SPSS

[To know how to correctly enter your data into SPSS in order to run a repeated measures ANOVA please read our Entering Data in SPSS tutorial.]

1. Click Analyze > General Linear Model > Repeated Measures... on the top menu as shown below:
   

   Published with written permission from SPSS Inc, an IBM Company.

2. You will be presented with the following screen:
   

   Published with written permission from SPSS Inc, an IBM Company.

3. In the "Within-Subject Factor Name:" replace "factor1" with a name that is more meaningful name for your independent variable.  In our example, we will call our within-subject factor name "time" as it represents the different times that we took CRP measurements from our participants (pre-training, pre + 2 weeks and post-training).

   Enter into the "Number of Levels:" box the number of times the dependent variable has been measured. In this case, enter "3", representing pre-training, pre + 2 weeks and post-training.

   Click the  button.

   Put an appropriate name for your dependent variable in the "Measure Name:" box. In this case we have labelled our dependent variable CRP.

   Click the  button.

   You will be presented with the diagram screen below:

   

   Published with written permission from SPSS Inc, an IBM Company.

4. Click the   button and you will be presented with the following screen:


Published with written permission from SPSS Inc, an IBM Company.

5. Transfer "Pre_Training", "Week2" and "Post_Training" into the "Within-Subjects Variables (time):" box by either drag-and-dropping or using the  button. If you make a mistake you can use the  and  buttons to reorder your variables.
   

   Published with written permission from SPSS Inc, an IBM Company.

6. Click the  button. You will be presented with the following screen:
   

   Published with written permission from SPSS Inc, an IBM Company.

7. Transfer the "time" factor from the "Factors:" box into the "Horizontal Axis:" box by either drag-and-drop or the  button.

   Click the  button. You will be presented with the following screen:

   

   Published with written permission from SPSS Inc, an IBM Company.

   Click the  button.

   You will be presented with the following screen:

   

   Published with written permission from SPSS Inc, an IBM Company.

Go to the next page for the remainder of the procedure and the SPSS output.