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Go ahead and add your other continuous covariates and demean them all as you add (even gender, so it will no longer be 0's and 1's).  That way the [1 0 0 0...0] contrast will be the mean for group A for the average value of everything else.

The [0 0 1 -1 0 0...0] contrast will not be impacted (much, if at all) by demeaning the other covariates.

Of course don't demean the first two covariates (your GroupA/GroupB covariates)

Cheers,
Jeanette

On Thu, Feb 7, 2013 at 11:26 AM, Jie Yu <[log in to unmask]> wrote:
Dear Jeanette
Back to my first question.....
Sorry, I just realise that I have other variables to control when trying to find different correlation between group A and B.
age , gender and total gray matter....
But I think I still don't need to demean any variables or use "D"?
Thanks
Jie-Yu