Yes, that is correct, though I think your matrix should be [.5 .5 -.5
-.5 0 0 0 0]. You should also remove the mean from the age (mean
computed from all subjects). Or even better, first test whether there is
a group difference in age slope with [0 0 0 0 .5 .5 -.5 -.5]. If there
is nothing that is significant, then re-run your analysis using the
Different Offset Same Slope (DOSS) model with this contrast [.5 .5 -.5
-.5 0].
doug
On 12/10/11 4:15 AM, Martijn Steenwijk wrote:
Dear all,
I'm relatively new with Freesurfer, but slowly getting more and more
used to it's great possibilities. To be 'sure', I've a question about
the design of a GLM.
I want to compare CT in Healthy Controls vs Diseased, and control for
age and sex. It appears to me that factors (eg sex) cannot be used as
covariate/variable, which forces me to model them as a separate class
although I'm not interested in sex differences. This brings me to the
following FSGD file:
# HcDis.fsgd
GroupDescriptorFile 1
Title HcDis
Class Hc_Male
Class Hc_Female
Class Dis_Male
Class Dis_Female
Variables Age
Input subjid1 Hc_Male 35
Input subjid2 Dis_Female 30
....
Then the difference between Hc and Dis, corrected for age and sex is
given by the contrast matrix
#Hc-vs-Dis.mtx
0.5 0.5 -0.5 -0.5 0 0 0 0 0 0 0 0
Is this correct?
Best,
Martijn
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