http://www.opensubscriber.com/message/freesurfer@nmr.mgh.harvard.edu/1250582...

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 greve@nmr.mgh.harvard.edu Phone Number: 617-724-2358 Fax: 617-726-7422

Bugs: surfer.nmr.mgh.harvard.edu/fswiki/BugReporting FileDrop: https://gate.nmr.mgh.harvard.edu/filedrop2 www.nmr.mgh.harvard.edu/facility/filedrop/index.html Outgoing: ftp://surfer.nmr.mgh.harvard.edu/transfer/outgoing/flat/greve/

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Ok, thanks! ________________________________________ 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: Wednesday, 6 August 2014 7:59 AM To: freesurfer@nmr.mgh.harvard.edu Subject: Re: [Freesurfer] effect of demeaning the age

Yes, I would demean before computing age*age. Is pubertal status a continuous variable? If so, then demean that too. doug

On 08/04/2014 06:53 PM, Sarah Whittle 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 greve@nmr.mgh.harvard.edu Phone Number: 617-724-2358 Fax: 617-726-7422

Bugs: surfer.nmr.mgh.harvard.edu/fswiki/BugReporting FileDrop: https://gate.nmr.mgh.harvard.edu/filedrop2 www.nmr.mgh.harvard.edu/facility/filedrop/index.html Outgoing: ftp://surfer.nmr.mgh.harvard.edu/transfer/outgoing/flat/greve/

---

Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer

The information in this e-mail is intended only for the person to whom it is addressed. If you believe this e-mail was sent to you in error and the e-mail contains patient information, please contact the Partners Compliance HelpLine at http://www.partners.org/complianceline . If the e-mail was sent to you in error but does not contain patient information, please contact the sender and properly dispose of the e-mail.

---

Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer

-- Douglas N. Greve, Ph.D. MGH-NMR Center greve@nmr.mgh.harvard.edu Phone Number: 617-724-2358 Fax: 617-726-7422

Bugs: surfer.nmr.mgh.harvard.edu/fswiki/BugReporting FileDrop: https://gate.nmr.mgh.harvard.edu/filedrop2 www.nmr.mgh.harvard.edu/facility/filedrop/index.html Outgoing: ftp://surfer.nmr.mgh.harvard.edu/transfer/outgoing/flat/greve/

_______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer