Michael,

Thanks for your response. It was helpful. As you suggested the "thickness-age correlation group difference" under DOSS is meaningless, so I have ignored that output.
I am currently left with a situation where under the DOSS model I see a large ageXthickness effect (controlling for group) and a nearly identical map under the DODS model. So far so good.
When I look at the group difference map, I see a large group difference for the DOSS model, and a LOT less for the DODS model. This would lead me to believe that the extra DOF in the DODS model is removing the group difference because my two groups have different ageXthickness slopes and when this is properly accounted for in the DODS model the group difference largely goes away. However, when I look  at the " does the thicknessXage correlation differ between groups" map I see almost nothing significant? I guess my hypothesis is that the slopes are different enough between my groups to wash away the group difference, but not large enough to show up as significant? It would be nice to be able to derive some per group ageXthickness slopes from clusters or ROIS to investigate this further, is this possible? I will probably also start looking into other models, just visually looking at different vertices it appears that most of the groups difference is in the older subjects (in terms of raw thickness values) with less/none difference in younger subjects.

One last aside. The demeaning of covariates still slightly confuses me. In a DOSS model it seems it wouldn't matter where you measure the intercept since both ageXthickness slopes are equal. In the DODS model, it doesn't seem to make sense to measure an intercept of two different slopes, it is more interesting to compare the slopes (in the case of different slopes you will get a different answer at every possible intercept). I am obviously a newbie to GLM and QDEC so sorry for any silly questions.

Chris  



On Fri, Mar 4, 2011 at 1:34 PM, Michael Harms <mharms@conte.wustl.edu> wrote:
Hi Chris,

There really shouldn't be a "thickness-age correlation group difference"
result with the DOSS model.  I have FS 4.1 (rather than 5.0 on my
system) but running an analogous model, I see that I do indeed get a
verbal "Description" for such a contrast.  However, if I compare that to
the "thickness-age correlation (accounting for group)" result, I see the
exact same map.  And if you look at the .mat files in the contrast
directory generated by qdec, you'll see that those two contrasts are
identical (i.e. [0 0 1]) (or at least they are for qdec with FS 4.1).

So, this appears to be a "bug" in the verbal descriptions that qdec
provides when using a DOSS model.

As to the group difference itself changing between the DOSS and DODS
models, that is totally to be expected.  Note that in the DODS model,
whether or not you demean (center) the age variable has a critical
impact on the manner in which you interpret the group contrast.

cheers,
-MH

On Thu, 2011-03-03 at 20:41 -0600, Christopher Bell wrote:
> 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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