On 1/28/14 7:03 AM, KimMJ wrote:
Dear Doug and experts
My question is about the choice of DODS or DOSS.
I want to compare cortical thickness between 2 groups (disease group
and control group), while controlling for the effect of age (nuisance
covariate).
I've already read through the mailing list about the issue of DODS and
DOSS designs.
If I understand correctly, I have to first use DODS (QDEC) in order to
find areas of significant group*age interaction by using the contrast
(0 0 1 -1).
Given there is no area of interaction that survived multiple
comparisons correction, I can go with DOSS (1 -1 0) using mri_glmfit
(command line) to test between-group differences in cortical
thickness, controlling for the effect of age. Am I right?
Yes
My another question is that 'MUST' I rerun DOSS model after confirming
no group*age interaction in DODS ? Or can I directly use the results
of DODS to look for between-group differences controlling for age (1
-1 0 0), since there is no group*age interaction ?
Is there much difference in results between the above-mentioned two
approaches?
Technically, yes, though the results will be different. How different,
no one knows. The reason I like going to DODS is that there are no
issues with the slopes being slightly different. It does not take much
time to re-run it.
I've found a reply by Doug from the mailing list: If there is no
interaction, then either DODS or DOSS is appropriate. DOSS will be
more powerful and a little more interpretable
(http://www.mail-archive.com/freesurfer@nmr.mgh.harvard.edu/msg33418.html).
From this context, what does the 'powerful' mean? Does DOSS yield
higher statistical values than DODS?
It means that you will have a higher degrees of freedom. All other
things being equal, it means more significant p values.
doug
Apology for the beginner's questions.
Thank you in advance for your help.
MJ
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