1. Is my understanding correct that you can not covary for baseline scores with repeated measures ANCOVA?

2. The analyses require a baseline in the time factor for within-subjects effects, so it can not be used as a covariate in between-subject effects in the same way as you can do in ANCOVA, is this correct, or there is another way to covary for a baseline for between-subject effect?

If I move pre-test scores to covariates it calculates 1-time point as a baseline and I lose the difference from the baseline to the first time point.

And main within-subjects effects become inaccurate.

3. Perhaps repeated measures ANCOVA analyses already contain adjusting for variance in the baseline scores?

4. If I use another covariate e.g age for repeated measures ANCOVA for between-subject effect, I observe that there are no significant interactions between the covariate and three treatment subgroups. Treatment sub-group differences did not change much, however, the main within-subjects effect time for all measures (the main result of this one-group repeated measures study) changes from being significant to non-significant.

Is my understanding correct that because age significantly interacts with the within-subjects effect, I need to first run it without the covariates and report the within-subjects effects and then run sub-group analyses with covariates and report the between-subject effect?

Do I also report about the age as a covariate interaction with treatment effects over time, and how to correctly interpret it?  

I will be very grateful for your advice and explanations.

Hi. I passed this on to the statisticians. Unfortunately, they don't have time to consult on issues that are more theoretical, due to several upcoming, time-sensitive assignments. Their involvement must necessarily be limited to statistical issues particular to the SPSS algorithms.

That said, one of them did get back to me with this:

"It is true that you can't put the same variable in as a covariate and a dependent in repeated measures. You can make a copy of the baseline measurement and include that as a covariate."

I hope that helps to some degree.

Viktoria - You sent me a message to an inbox I cannot for some reason reply on. Please re-send to marcantr@us.ibm.com. Thanks and sorry for the inconvenience.

BTW - My reply was going to be that you need to use the SD of the differences, not the SD of one of the groups, for the test you were referring to.