Dear Peter,
if I get you right you did the following:
24 subjects overall
2 groups
2 conditions for every subject
You modeled the subject factor as main effect on 2nd level.
Then question is know how you modeled the factors group and condition.
One good way to do this would be to model the interaction between
group and condition.
If you model your desing this way, you get 28 columns. Column 1-24 are
the subjects. Column 25 is condition 1 for all subjects of group 1.
Column 26 is conditon 2 for all subjects of group 1. Column 27 and 28
are condition 1 and 2 for the subjects of group 2. If you are
interested in condition 1 > condition 2 in group 1 your contrast
vector would look like [zeros(1,24), 1, -1, 0, 0]. Condition 1 >
condition 2 in group 2 would be [zeros(1,24), 0, 0, 1, -1]. These are
simple effects, however. If you are interested in the main effect,
your contrast vector would be [zeros(1,24), 1, -1, 1, -1].
Hope that helps,
BaSti
Zitat von Peter Kirsch <[log in to unmask]>:
> Dear colleagues,
>
> I would be very grateful if anybody would have an idea how to solve the
> following problem:
>
> I set up a 3 factorial mixed effects design with 24 subjects using the
> flexible factorial design option in SPM5. I have one wihtin-subject factor
> (2 levels: condition) and one between-subject factor (2 levels: group). I
> modelled the subject effect as the first factor. The design matrix was
> estimated but when I try to calculate contrasts, SPM tells me that my
> contrasts are invalid.
> The first contrast is testing condition 1 > condition 2 and I defined it as
> follows:
> [zeros(1,24), 1, -1]
> Any contrast with less then 25 columns is working but whenever I try to
> define a contrast for my experimental factors, I get an error message.
> I am quite sure that there is an answer to this in the depth of the archive
> but unfortunately I could not find it.
>
> Thanks in advance for your invaluable support,
>
> Peter
>
> --
> Peter Kirsch, PhD
> Division for Imaging in Pychiatry
> Central Institute of Mental Health
> J 5
> D-68159 Mannheim, Germany
>
>
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