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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--
Douglas N. Greve, Ph.D.
MGH-NMR Center
greve@nmr.mgh.harvard.edu
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