I have done a RCT (randomized control trial) of an intervention (which is of 6 week duration) versus control. The outcome measure (an exercise test) has been tested pre-intervention (Pre), immediately after (Post-1), 6 months after (Post-2), and one year after intervention (Post-3).
I have done a repeated measures ANOVA in SPSS. There is no group difference at Post-3. However, looking at the means/plots at each time point there appears to be a difference between the groups at Post-1.
How would I test to see if there is a significant group difference at Post-1?