Hi Clara,
the SPC (or any percent change) could be problematic, as the base value (either the volume at the baseline timepoint, or the average volume) will change depending wich interval you look at. You could check if the rate (mm/time_unit) or mm^3/time_unit is changing.
I am not sure how your study is setup, so I cannot really recommend anything here (e.g. when did the intervention happen exactly).
Generally I would recommend a piecewise linear model with node at the intervention time in LME (mixed effects).
Best, Martin
On 21 Apr 2017, at 17:15, Clara Kühn ckuehn@cbs.mpg.de wrote:
Dear FreeSurfer experts,
I have a longitudinal design with 3 groups and 3 time points. For one group the intervention did not work so for now I would like to look at 2 groups only. Can I do this with the Longitudinal Two Stage Model by looking at the spc from tp1 to tp2 and then looking at the spc from tp2 to tp3?
Are there statistical reasons that forbid this procedure of comparing the change across time between groups?
Thanks for your help! Clara
-- Clara Kühn, Phd Student
Max-Planck-Institute for Human Cognitive and Brain Science Department of Neuropsychology Stephanstrasse 1A 04103 Leipzig, Germany
Phone: +49 341 - 9940 2271 Fax: +49 341 - 9940 2260 Web: www.cbs.mpg.de E-Mail: ckuehn@cbs.mpg.de
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