Dear FSLers,
I am writing to confirm the design matrix I have been using to investigate
changes in activation during a block design task, as I have not been able to
find anything specifically on FSL's web site about my particular design, and
I am a bit confused as to how I should proceed, and I haven't managed to
find anything specifically about this problem on the email list. The answer
should be quite simple, but I want to make sure that I'm doing things correctly.
The problem is as follows. I am performing a higher level analysis where
there are two groups (patients and controls) with about 20 subjects in the
patient group and 35 subjects in the control group. The subjects do not
differ significantly in age nor in sex distribution, but I would like to
include sex and age as covariates to minimize effects that might be
accounted for by these variables. So, at the moment I am creating a design
matrix that looks as follows:
Group control patient age sex
1 1 0 21 1
1 1 0 24 -1
1 1 0 35 -1
1 1 0 42 1
1 0 1 23 1
1 0 1 43 -1
1 0 1 22 1
I am also orthogonalizing the age and sex variables with respect to EV1 and
EV2 (controls and patients). The groups are all assigned to one since I
cannot estimate separate group variances when covariates are added to my
model. So then, in my contrasts, I would like to look at differences in
activation between patients and controls, and I do this:
Contrast control patient age sex
ctrl>pat 1 -1 0 0
pat>ctrl -1 1 0 0
avg ctrl 1 0 0 0
avg pat 0 1 0 0
with no F tests. Is this correct? Am I correct in assuming that the test
I'm performing with controls > patients and patients > controls is still an
F test because age and sex are included in the model, or do I have to
specify these explicitly? If I do the F test, how can I determine
directionality of the contrast?
If anyone could please confirm that I have gone about this in the right (or
wrong!) way, I would very much appreciate it.
Thank you so much in advance!!
Liberty Hamilton
Laboratory of Neuro Imaging at UCLA
http://www.loni.ucla.edu
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