FreeSurfers,

I have been analyzing my qdec data in version 5.0 and have some interesting
although somewhat confusing results. Basically I have run a very simple
analysis with DODS and DOSS. My discrete factor is group and my covariate is
age, about as simple as can be.

When I look at the results for DODS I get:

thickness-age correlation (accounting for group)---result: much of brain
significant
group difference (I assume controlling for age, but it doesn't say
explicitly)--result: one small roi significant
thickness-age correlation group difference--result: one small roi spatially
adjacent to group difference roi

When I run DOSS I get:

thickness-age correlation (accounting for group)---result: much of brain
significant
group difference (I assume controlling for age, but it doesn't say
explicitly)--result: much of brain significant
thickness-age correlation group difference--result: much of brain
significant

I am mostly surprised by how much larger the (group difference), and the
(thickness-age correlation group difference) increase with the DOSS method.
I am also not quite how to interpret the thickness-age correlation group
difference in DOSS. I was thinking the DOSS method constrained both groups
to have the "same slope" and so I was expecting to get nothing for
difference in thickness-age correlation difference by group; isn't this
suggesting my two groups have significantly different ageXthickness slopes
even though they are constrained to have the same slope by the DOSS method?
It would almost make more sense to me if the results were reversed between
DOSS and DODS. If I had a large thicknessXage correlation group difference
using the DODS method which allows for different slopes. Thanks for any
enlightenment.

Chris Bell
University of Minnesota
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