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?
Best Answer
Although the answer could be very extensive explaining all aspects of a repeated measures design I will keep it short and answer the question directly. I think, however, that it is wise to do a bit more study on your own to be able to comprehend the answer completely.
You have to look whether the interaction between the group variable (control vs. intervention) and the factor variable (the measurements over time). Although you can proceed to the next step either way it is advised that you only do so in case this step is significant.
In the SPSS repeated measures menu you click options >> select interaction term >> drag to the right column >> click continue >> click ok. You now get descriptive statistics for each group for each time interval. Using the confidence intervals you can determine whether or not the two groups differ on a certain time point.
The syntax you have to add in the second step is: