I would like to perform a two-way repeated measures ANOVA test for my research. In my data, I have six time periods and two groups (intervention and control group) - I am interested in the interaction effect time*group. In the control group, there is no treatment at all and in the intervention group, the treatment takes place after period 3 (i.e. there are three periods before and three periods after the treatment).
I can run the model in SPSS, however I don't quite understand yet how the timing of the treatment influences this model/the interpretation. In the model, I do not specify when the treatment takes place, but surely it must make a difference when it takes place. Should I leave out the first two time periods in this case, so that I only have a status quo and three post-treatment observations? Or do I only need to take into account the timing of the treatment for the interpretation?