I have a model in which two groups are measured on one DV at pre- post- and 3 months following an intervention (the control group did not receive the intervention). I have an interaction effect, which I believe implies that the combination of group and time together influence the DV, and yet the same SPSS ANOVA output shows no main effects. How should I interpret the data? Is there an easy way to do further analyses with SPSS?
Solved – Interpret significant interaction but no main effect in repeated measures ANOVA
anovainteractionmixed modelspss
Best Answer
Here is an example where there's really no interaction. That means that the effect of factor B is additive and is the same at each level of factor A (the difference between the blue point and the red point is the same across all levels of factor A).
The lines joining the means provide the so-called interaction plot. If there is an interaction then we should not be interested in the main effects (but be careful about the meaning of "no interaction").