Hi Jorge,
Thanks again for the detailed and very helpful explanation !
I thought that all statistics should better be performed with LME in
order to give more credibility to final results. But also it brings more
trust if in the article statistics are performed with the same method.
Anyway - we will present LME results.
Have a great day !
best wishes,
Alex.
Le 2/13 0:18, jorge luis a écrit :
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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