Dear Varun,
you may find some help in the CAT12 manual at page 47:
http://www.neuro.uni-jena.de/cat12/CAT12-Manual.pdf
This example is for two groups, but it should be quite straightforward to extend it to the 3rd group.
T-test
For Time 1 > Time 2 in Group A 1 -1 0 0
For Time 1 > Time 2 in Group B 0 0 1 -1
For Time 1 > Time 2 in Groups A and B 1 -1 1 -1
For Time 1 > Time 2 & Group A > Group B (interaction) 1 -1 -1 1
F-test
For any differences in Time in Group A eye(k)-1/k
For any differences in Time in Group B [zeros(k) eye(k)-1/k]
For main effect Time [eye(k)-1/k eye(k)-1/k]
For interaction Time by Group [eye(k)-1/k 1/k-eye(k)]
For main effect Group [ones(1,k)/k -ones(1,k)/k ones(1,n1)/n1 -ones(1,n2)/n2]
Effects of interest (for plotting) [eye(k)-1/k zeros(k); zeros(k) eye(k)-1/k]
However, the contrast for group comparison at baseline may artificially inflate your results. More information about that issue can be found here: https://doi.org/10.3389/fnins.2019.00352
Best,
Christian
On Tue, 12 Apr 2022 02:13:40 +0100, Varun Arunachalam Chandran <[log in to unmask]> wrote:
>Dear SPM experts,
>
>I work on a longitudinal VBM analysis consisting of 3 groups (2 different patient groups and 1 healthy control group) with time point 1 and time point 2. With this dataset, I would like to look at the longitudinal grey matter volume differences between three groups (changes from baseline to follow-up). In addition, I would like to do a cross-sectional analysis between the 3 groups with time point 1 and time point 2 separately. Could you please suggest how I can apply flexible factorial design (or an alternative statistical model) to construct a design and contrast matrix for these two different types of analysis.
>
> thanks,
>
>kind regards,
>Varun
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