On Tue, Mar 6, 2012 at 5:10 PM, Bruce Fischl <fischl@nmr.mgh.harvard.edu> wrote:

do you have a big outlier in the thickness measures? I didn't see one. It seems unlikely that all your subject have a defect in that spot

On Tue, 6 Mar 2012, Douglas N Greve wrote:

It means that any comparison depends on age. Eg, if you compare them at
an age where the lines cross, you will see no effect. If you compare
them at age=0 (what you were doing before), then you see a big effect.
Some statisticians will say that you cannot do the comparison in the
presence of an interaction.
dogu

Allie Rosen wrote:

So if my findings show that each group ages differently, does that
mean I can't compare them at all because I can't regress out age? Or
is there some way of comparing the groups despite this difference in
aging patterns?

Thanks again,
Allie

On Tue, Mar 6, 2012 at 4:44 PM, Douglas N Greve
<greve@nmr.mgh.harvard.edu <mailto:greve@nmr.mgh.harvard.edu>> wrote:

It might, don't know

Allie Rosen wrote:

Hi Doug,

Yes, I think there actually may be a topologic defect in this
area, although not directly where the cluster is (attached).
Could this be the cause of the thickness result? If I go back
and try to repair the defect, will that make the results more
typical and, more importantly, correct?

Thanks again,
Allie

On Tue, Mar 6, 2012 at 3:57 PM, Douglas N Greve
<greve@nmr.mgh.harvard.edu <mailto:greve@nmr.mgh.harvard.edu>
<mailto:greve@nmr.mgh.harvard.edu
<mailto:greve@nmr.mgh.harvard.edu>>> wrote:

Hi Allie, yes, it means that each group is affected
differently by
age. The controls are increasing their thickness with age
in this
area (which seems strange, usually you expect general decreases
with age). You might check the quality of the surfaces in
this area.
doug

Allie Rosen wrote:

Hi Doug,

Thanks for the response. I've actually already run an
interaction contrast and the same area is still significant
(attached). I'm not sure how to interpret this either,
unfortunately. Does it say that both groups are affected
significantly differently by age? Can you tell me what it
means? I'm having problems trying to do this myself.

Thanks very much,
Allie

On Tue, Mar 6, 2012 at 3:47 PM, Douglas N Greve
<greve@nmr.mgh.harvard.edu
<mailto:greve@nmr.mgh.harvard.edu>
<mailto:greve@nmr.mgh.harvard.edu
<mailto:greve@nmr.mgh.harvard.edu>>
<mailto:greve@nmr.mgh.harvard.edu
<mailto:greve@nmr.mgh.harvard.edu>

<mailto:greve@nmr.mgh.harvard.edu
<mailto:greve@nmr.mgh.harvard.edu>>>> wrote:

Allie, to be more precise, you have selected
control<patient
regressing out the effects of age. This second part
is the key.
If you were to trace the regression lines back to
age=0 (the
meaning of "regressing out age"), then
control<patient by a
wide
margin. But you have a problem here in that the
regression
lines
are crossed. This means that you have an interaction
between age
and patient (no interaction means that the lines
would be
parallel). An interaction means that you cannot
assess whether
there is a difference in the thickness because this
difference
changes depending upon the age. One thing you can
try is to
actually compute an interaction contrast (something
like [0 0 1
-1]). If it is significant in this area, then
there's not
much you
can do (though it may be interesting in itself). If
it is not
significant, then you can switch to a DOSS model which
forces the
lines to be parallel and makes the distance between the
lines be
independent of age. Does this make sense?
doug

Allie Rosen wrote:

Hi All,

I'm having trouble reconciling the cluster
colours with the
graph that can be made when you load the group
descriptor file
and click on a point. I've attached an example.
In this study, I'm comparing thickness in a
patient and
control group, with age regressed out. The contrast
I've used
means that blue means control<patient. Therefore,
patients are
thicker than controls. But in the graph,
patients don't
seem
to be thicker. I've seen the same results with
red clusters
(i.e. they should mean that controls are thicker but
the graph
shows otherwhise). Please let me know how I can
interpret my
findings given the graph. Which group is
actually thicker?

Thank you,
Allison

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