Hi Alex

Certainly, for such a simple design matrix, only two repeated measures per subject with a small inter-subject variability in between-scan time interval and a very small number of subjects with a single time point the results from both procedures should not be very different. Furthermore, by using TSM you have the additional possibility of applying cluster-wise simulation-based multiple comparison procedures implemented in Freesurfer. LME must be used to address more complicated designs with time-varying covariates, variable timing across subjects, variable drop-out rate across measurement occasions, different number of repeated measures across subjects (even a single time point) and subjects with more than three repeated measures.

For your intra-group analyses using LME you simply need to test whether group-specific slopes of change are different from zero. You don't need to fit the model again for this. For the reference group you should simply test the coefficient of the time variable:

CM.C = [0 1 0 0]

and for the other group the contrast is given by the sum of the previous coefficient and the coefficient of the interaction term:

CM.C = [0 1 0 1]

Best
-Jorge



De: Alex Hanganu <al.hanganu@yahoo.ca>
Para: FS Mailing List <Freesurfer@nmr.mgh.harvard.edu>; Jorge L. Bernal-Rusiel <jbernal@nmr.mgh.harvard.edu>
Enviado: Martes 12 de febrero de 2013 18:29
Asunto: [Freesurfer] LME - contrast

Dear Jorge,

For the statistical analysis of longitudinal data we used the "Two Stage Model" (TSM) and the "Linear Mixed Effects".

Our intergroup LME results (2 groups, 2 time points) had many similarities with TSM results. Then with TSM we analysed the intragroup changes (one group, and 2 time points). We tried to do the same analysis with LME but the results are very different, so I suppose that the contrast was incorrect.

For the intergroup analysis (2 groups x 2 time points) I used lme_mass_fit_vw and CM.C = [0 0 0 1];

For the intragroup (1 group x 2 time points) I also used lme_mass_fit_vw but CM.C = [0 1];

Can you please tell me if the CM.C is correct ? or is there something else I should change ?

Thanks,

Best regards,

Alex.

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