Perhaps someone else has better insight then me, but I've never been confident that statistics with repeated measures ANCOVAs are accurate. I've done a number of searches and haven't found a definitive source on them. When I talk to statisticians, there doesn't seem to be a concensus on how to compute repeated measure ANCOVAs. In tests of multiple program, I got different results -- suggesting that the field has not settled on a method.

For your particular question, you can use a two-sample t-test and avoid the issues of repeated measure ANCOVAs. Subtract the pre-/post- training for each subject, then subtract the RT values for pre-/post-training for each subject. Then you can use the RT differences as covariates in a two-sample t-test.

Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Office: (773) 406-2464
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On Wed, Jan 11, 2012 at 8:11 AM, Torsten Ruest <[log in to unmask]> wrote:

Hi there,

I would like to do a repeated measures analysis but I am not quite sure whether my model gives me the answers to my questions. If someone could comment on the design, I'd highly appreciate this.

I have 2 groups (patients vs controls), and 2 time points for each (after/before training). In addition, I have a covariate (RT) which I want to use to see whether (1) time (training) interacts with the covariate (RT) within each group (contrast 4: patients, contrast 5: controls), and whether (2) time (training) interacts with the covariate (RT) across groups (contrast 6). (Note RTs have been demeaned across all conditions and then split into respective EVs).

It's my 1st repeated measures design, so I hope this makes any sense.

Thank you very much in advance for any comments.

Cheers,

Torsten