Hi friendly SPM users,
I want to run an ANCOVA in SPM2 to control for gender across 2 groups.
When I try, my design matrix has a constant mean column, which my supervisors tell me should not be there because "if the input (as we have) is a contrast image i.e. a subtraction of conditions, then they already have effectively removed differences in the mean activity per subject at the first level (they are 'difference scores'), so we don't need to add the constant term at the second level".
So my design matrix has 4 columns when it should only have, 3 due to the modelling of the group means.
Can anyone provide input? Thanks for your time.
Garret (Edinburgh)
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