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 <kas...@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 > > > The information in this e-mail is intended only for the person to whom it is > addressed. If you believe this e-mail was sent to you in error and the e-mail > contains patient information, please contact the Partners Compliance HelpLine > at > http://www.partners.org/complianceline . If the e-mail was sent to you in > error > but does not contain patient information, please contact the sender and > properly > dispose of the e-mail. _______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer