Dear Franz,
I know this is almost invisible, but for defining the conditions you have to use a single quote after the bracket. I have now indicated this in red color in the manual and added a note. If you forget the single quote the conditions are mixed up and the design matrix is wrong.
The first 4 columns are your TP1 and TP2 for group A and B.
-1 1 -1 1 TP2>TP1 & Group B>A etc.
Regards,
Christian
____________________________________________________________________________
Christian Gaser, Ph.D.
Departments of Psychiatry and Neurology
Friedrich-Schiller-University of Jena
Jahnstrasse 3, D-07743 Jena, Germany
Tel: ++49-3641-934752 Fax: ++49-3641-934755
e-mail: [log in to unmask]
http://dbm.neuro.uni-jena.de
On Thu, 6 Oct 2011 09:58:22 +0100, Franz Riederer <[log in to unmask]> wrote:
>Dear Christian and others
>
>I want to comapare grey matter changes over time (2 time points, termed A and B) in 2 groups of subjects (termed P and G)
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>For that purpose I used the longitudinal VBM8 batch to process T1 volumes.
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>I am particularely interested in changes within group P over time.
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>I built up a flexible-factorial design with the factors "subject", "group" and "time", defining the interaction "group X time" an the main effect "subject", as recommended in the VBM8 manual on pp 20-22.
>
>The question is how to define contrasts.
>Please have a look at the design in the attached PDF.
>
>My approach would be the following:
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> 1 -1 0 GM decrease within group P, time A versus B
>-1 1 0 GM increase within group P, time A versus B
> 0 1 -1 GM decrease within group G, time A vs. B
> 0 1 -1 GM increase within group G, time A vs. B
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>Finally what would be the ideal contrast for an interaction analysis group X time ?
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>I would be very grateful for any suggestions.
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>Best regards
>
>Franz
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