Dear FreeSurfer experts,

I would appreciate a confirmation regarding correctness of my approach.

I am trying to run an LME model for three groups (group1=controls, group2 and group3) and two time points.

I followed the tutorial on https://surfer.nmr.mgh.harvard.edu/fswiki/LinearMixedEffectsModels https://surfer.nmr.mgh.harvard.edu/fswiki/LinearMixedEffectsModels and adapted the four group example. However, I am not 100% sure if my adaption is correct and would appreciate your input.

The design matrix for the model contains the following columns:

1) Intercept

2) time (tij)

3) one for Group2 and zero otherwise

4) column 3) .* time

5) one for Group3 and zero otherwise

6) column 5) .* time

To test the null hypothesis of no group differences in the rate of change over time among the three groups I applied the following contrast:

0 0 0 1 0 0

0 0 0 -1 0 1

Is this contrast correct?

In a second step, I would use the additional three contrasts to identify which groups were different:

0 0 0 1 0 0 and 0 0 0 0 0 1 and 0 0 0 -1 0 0 1

I really appreciate your help!

Best wishes

Sarah