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
Date:06/03/2014 6:15 PM (GMT-05:00)
To: FS Mailing List
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.