Hi Cleo, See the following page created by Jeanette Mumford for a helpful exposition of how mean centering affects your data:

http://mumford.fmripower.org/mean_centering/

As for your DOSS model, there is a "bug" in how QDEC describes the contrasts in that case. See the following posts:

http://www.mail-archive.com/freesurfer@nmr.mgh.harvard.edu/msg08912.html http://www.mail-archive.com/freesurfer@nmr.mgh.harvard.edu/msg16888.html

Follow-ups to the later post also touch on the issue of demeaning as well.

cheers, -MH

On Fri, 2011-05-06 at 00:30 -0400, cgomez@bidmc.harvard.edu wrote:

Hi Freesurfers,

I’ve been using freesurfer5, qdec 1.4 and I have some questions about it.

1-I’ve used age as a nuisance variable in my analysis (DODS). When I use demeaned age I obtain a different result if I use ‘real’ age value, which is your the recommendation?

2-When I use DOSS model, which is the meaning of the question: ‘Does the thickness-performance correlation differ between A2 and A3?’ If you are using DOSS model both groups have the same slope and the same correlation value, isn’t it? Moreover, the results (areas) that I obtain are the same (but with colors reversed) as those obtained in ‘Does correlation between thickness and performance, accounting for group, differ from zero?’

These are the contrasts:

lh-Avg-thickness-perf-Cor -----------------------

Does the correlation between thickness and perf, accounting for group, differ from zero?

Nuisance factors: Gender Age

0.000 0.000 1.000 1.000 0.000;

lh-Diff-bed-cnt-Cor-thickness-perf -----------------------

Does the thickness--perf correlation differ between A2 and A3?

Nuisance factors: Gender Age

0.000 0.000 1.000 -1.000 0.000;

When the order Xg.dat is: group1 group2 performance gender age.

So, the contrast not seem correct at all..??

Thanks in advance for your help.

Cleo.

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