http://www.opensubscriber.com/message/freesurfer@nmr.mgh.harvard.edu/12505825.html

2) Do I have to include age covariate in that model even if I clearly
> matched the age distribution?
>
Technically, you do not, but it should be a good idea to do so. You can
try one of these things: (a) use DOSS (fits the same slope to both
groups), or (b) stay with DODS and demean the ages. This is the same as
what you are doing already, but it tests for the difference at the mean
of your group instead of at 0.

Best regards,
Amirhossein Manzouri

On Tue, Aug 5, 2014 at 12:53 AM, Sarah Whittle <swhittle@unimelb.edu.au> wrote:

Hi,

I have a follow-up question about demeaning, specifically applied to running linear mixed effects models.

For example, we have a design matrix X with the following columns:
intercept (all ones)
age
gender
age*age
pubertal status

If we want to look at the effect of age*age, controlling for pubertal status and gender [0 0 0 1 0 ], should age be demeaned prior to calculating the interaction term? I presume so, because if not, age*age will be highly correlated with age. I also presume that the covariate pubertal status should be demeaned?

Thanks,

Sarah
________________________________________
From: freesurfer-bounces@nmr.mgh.harvard.edu [freesurfer-bounces@nmr.mgh.harvard.edu] on behalf of Douglas N Greve [greve@nmr.mgh.harvard.edu]
Sent: Tuesday, 5 August 2014 2:09 AM
To: amirhossein manzouri; free surfer
Subject: Re: [Freesurfer] effect of demeaning the age

check out this web page
http://mumford.fmripower.org/mean_centering/

On 08/04/2014 12:01 PM, amirhossein manzouri wrote:
> Hi,
> I am looking at group differences between patients(20) and
> controls(n=20). I get significant results when I use Age, while I get
> no significance using Demeaned age both with DODS, also the pattern of
> significance is different. Would you please explain why?
>
> Best regards,
> Amirhossein Manzouri
>
>

--
Douglas N. Greve, Ph.D.
MGH-NMR Center
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