Hi Meghan, you will need to use an F-contrast. Choose any two of those contrasts and put them in a single file with two lines. Before you do that, you should look for an interaction between your groups and your covariate with another F-contrast:
0 0 0 1 0 -1
0 0 0 1 -1 0
with the DODS model. If you don't see an effect anywhere (probably the case), then re-run the model with DOSS and contrast
1 0 -1 0
1 -1 0 0
The DOSS uses a single regressor for the covariate which makes comparisons across the group more meaningful. If you do have an interaction, then there's not much you can do because the results are hard to interpret.

doug

On 4/2/12 2:25 PM, McGwier, Meghan (NIH/NIMH) [F] wrote:

Dear experts,

I’ve been reading through the tutorial for group analysis, but I am still a little confused about the contrasts and want to make sure that my contrast is set properly for what I’m interested in looking for. I’ve made my fsgd file, which contains three groups: ADdiagnosisGroup1, ADdiagnosisGroup2, ADdiagnosisGroup3, and one continuous variable: symptom duration. I want to test if thickness differs between groups controlling for symptom duration. I set my contrasts to: [1 0 -1 0 0 0], [1 -1 0 0 0 0], [0 1 -1 0 0 0]. Since there isn’t an fsgd example for 3 groups (1 factor, 3 levels), and one covariate, I wasn’t sure if this is correct.

Thanks in advance,
Meghan

Meghan McGwier
Postbaccalaureate IRTA Fellow
Molecular Imaging Branch
National Institute of Mental Health
Phone: 301-451-2038
Fax: 301-480-3610
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