Hi Kasper,

your design matrix X should have as many rows as there are measurements. Say if you have m subjects with n measurements each, it will be m*n rows. If different subjects have different numbers of measurements, that's also OK; in such cases the number of rows per subject in X will also differ.

Then, X should contain the following columns:

1. intercept this column should be all ones

2. time from baseline if you have two time-points for each subject, this column should look as follows: 0 follow-up-time_subj1 0 follow-up-time_subj2 0 follow-up-time_subj3 etc. here, follow-up-time is the difference in time between time-point 1 and 2, i.e. (time-point 2 minus time-point 1).

3. group indicator 0 for controls, 1 for patients

4. time x group interaction multiply the values of columns 2 and 3

Further, it is important that the rows of X are ordered according to time for each subject. In your case with two measurements for each subject (if I understood correctly), this means that for the rows of X, time should be the faster changing index, and subject the slower changing index. This results in a sequence like subj1_time1 subj1_time2 subj2_time1 subj2_time2 subj3_time1 etc. Needless to say, this must be the same in the ordering of your data variable Y.

Hope this helps,

Kersten

Am 08.08.2017 um 14:52 schrieb Kasper Jessen kasper@brejes.com:

Dear FreeSurfer,

I have a longitudinal study (patients and controls) with two time points (i.e., baseline and six-weeks follow-up). I would like to use the LME model as it can handle unequal timing and different number of time points across subjects due to missing data.

I would like to test if there is an effect of (a) time (b) group and (c) group x time interaction.

However, i am trouble with my design matrix X:

(1) How would i write X?

Thanks in advance Best regards Kasper _______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer

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