Hi Garret,
Not sure if I understand correctly your question, and I am not
familiar with SPM2 (I use SPM5/8). If you obtain your contrast images
at 1st level analysis (i.e withon subject, contrasting one condition
to another), then at second level analysis, when you ask questions
about a group of participants (or compare groups), I think you should
expect to get the constant columns. Now they model the group mean(s).
Yannis
Sent from my iPhone
On 22 Apr 2010, at 09:26, "Garret O'Connell"
<[log in to unmask]> wrote:
> 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)
|