How to interpret an interaction effect in SPSS output for a repeated measures anova with between and within group factors?

Question

I have conducted a repeated measures ANOVA, with 2 factors:

• One between factor (Ex_control: exercise intervention versus control)
• One within factor (time: pre/post intervention).

Questions

• Should I first refer to the table labelled within-subject effect and specifically the box labelled time*Ex_control?
• Does this table show whether or not there was a significant between or within group change as a result of the intervention?

Answer 1

The line labelled time*Ex_control will tell you whether there is a statistically significant time by group interaction.

In the case of pre-post treatment-control design, this is often the effect of greatest interest. It is relevant to assessing whether the effect of time varies between groups. For example, if it is statistically significant you would then look at the direction of the sample means to see whether for example the group that received the exercise group improved more over time than the control group.

This can be contrasted with the two main effects. The main effect of time will test whether there is a significant change over time averaged across both groups. The main effect of group will test whether on average one group scored higher on the dependent variable than the other.