Dear Doug and Donald Thank you for your kind directions. 1) When I perform DOSS using the contrast (1 -1 0), the significant areas of cortical thinning in patients relative to controls (i.e., temporo-occipital cortex) were similar but more widespread in comparison with those in DODS analysis at the same threshold of CWP < 0.05.2) According to your suggestion, I've tried DODS with and without demeaning the covariate (age in my study) and observed virtually same results (to my eyes) between the 2 approaches. I'd appreciated very much your patience with my basic questions.
Min
Date: Thu, 30 Jan 2014 13:16:42 -0500 From: mclaren.donald@gmail.com To: freesurfer@nmr.mgh.harvard.edu Subject: Re: [Freesurfer] A question regarding DODS or DOSS
You might also want to read about mean centering and same or different slopes here: http://mumford.fmripower.org/mean_centering/
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On Thu, Jan 30, 2014 at 12:03 PM, Douglas N Greve greve@nmr.mgh.harvard.edu wrote:
Technically, you are on safe ground using DODS in that I don't think a
reviewer would object. However, I would be a little worried as to
whether the group difference is real. Does DOSS start to look like DODS
if you lower the threshold? Try running DODS with and without demeaning
the covariates to see whether it changes much.
doug
On 01/28/2014 07:52 PM, KimMJ wrote:
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: greve@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
<http://www.mail-archive.com/freesurfer%40nmr.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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