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.hang...@yahoo.ca>
>Para: FS Mailing List <Freesurfer@nmr.mgh.harvard.edu>; Jorge L. Bernal-Rusiel
><jber...@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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