Dear Doug
Thank you for the prompt reply.I want to make sure: If there is no group*age
interaction, is it safe to report the result of group differences using DODS (1
-1 0 0 )?Or is it more appropriate (or mandatory) to further use DOSS to look
for group differences (1 -1 0)?The reason why I ask you is that the results of
group differences using DODS is satisfactorily significant to support my
hypothesis.
Sorry for bothering you again with my silly question.Thank you.
Min J.
Date: Tue, 28 Jan 2014 12:10:42 -0500
From: gr...@nmr.mgh.harvard.edu
To: freesurfer@nmr.mgh.harvard.edu
Subject: Re: [Freesurfer] A question regarding DODS or DOSS
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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