Hello,
I have a multi-site study in which I'd like to compare change in fMRI data between two groups of subjects (patients and controls) over time (time 1, time 2), as well as the main effect of time and main effect of group. Although some of my subjects only have one time point, I'd like to use all scans in my analysis. In my model, I need to control for age at baseline, sex, scan site and duration of time between time points.
It seems that the modified Sandwich Estimator may be best suited for this analysis. However, I'm having some trouble understanding the user guide on FSL's website. The SWE help pages in the SPM/Matlab literature are similarly confusing. Therefore, any help you can provide would be greatly appreciated. My specific questions are as follows:
1. Is the the modified Sandwich Estimator appropriate for this analysis?
2. In the design.sub file, do I include only 3 columns (indicating subject ID, time point and group)? And place age at baseline, sex, scan site and duration of time between time points in the design.mat file?
3. For subjects with only one time point, how do I enter duration of time between time points?
4. Do I need to mean center any variables?
Thanks in advance for your guidance,
Lara
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