Hi Alex, The time distance does not matter. It's best to use a longitudinal method (both for the image processing, to reduce noise, and for the statistics, to gain power).
Your design seems to be missing a control group (unless one off your groups is placebo). In your design you can compare changes across the two groups. But if you look at a single group and detect longitudinal change you will not know if it is the drug or something else that caused it. Best Martin Sent via my smartphone, please excuse brevity. -------- Original message -------- From: Alexandru Hanganu <al.hang...@yahoo.ca> Date:06/03/2014 6:15 PM (GMT-05:00) To: FS Mailing List <Freesurfer@nmr.mgh.harvard.edu> Subject: [Freesurfer] Advice - best method, longitudinal vs. cross-sectional Hello Everyone, could someone please give us an advice about which method you consider is the best for our study ? we have two groups with MRI at Time 1. Each group received medication. After this we performed another MRI at Time 2 after 2 weeks. The best method for this study is a longitudinal one or a cross-sectional GLM ? We consider that the distance between the time points is too small, and the longitudinal method is not the best choice. Hence, this study should be treated as a cross-sectional one. In this case we think about performing a simple GLM with the contrasts: 0.5 0.5 0.5 0.5 or 1 -1 -1 1 for the groups: 1) grp 1 time 1 2) grp 1 time 2 3) grp 2 time 1 4) grp 2 time 2 we are searching to see whether medication had any impact on the cortical morphology in each group and between the groups. Thank you ! Best regards, Alex.
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