Hi Donald and all,

thank you very much for your answer.
The website you suggested was great to correct some mistakes in the mean centering I was still doing (like demeaning within groups in the second case).

However, I still have some doubts about the design which is most suitable for my work. 
In my case the covariate is the task performance, a continuous variable which is constant for controls (at the maximum value: everyone perform well) and generally  increases with the increasing of the activation (and vice versa) for the two group of patients (one group with mild impairment has high activation and acceptable performance, the other has bad performance and small activation). 
Considering this, which model should I choose? What should I look at in taking this decision?  

Thank you very much-


Giulia



> Date: Mon, 23 Jul 2012 15:36:19 -0400
> From: [log in to unmask]
> Subject: Re: [FSL] GLM - ANOVA with covariates
> To: [log in to unmask]
>
> The designs are different and have different assumptions about the
> data. The following website has a great explanation of your two
> models:
>
> http://mumford.fmripower.org/mean_centering/
>
> Best Regards, Donald McLaren
> =================
> D.G. McLaren, Ph.D.
> Research Fellow, Department of Neurology, Massachusetts General Hospital and
> Harvard Medical School
> Postdoctoral Research Fellow, GRECC, Bedford VA
> Website: http://www.martinos.org/~mclaren
> Office: (773) 406-2464
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>
> On Mon, Jul 23, 2012 at 5:52 AM, Giulia Preti <[log in to unmask]> wrote:
> > Hi there,
> >
> > I'm running an fMRI analysis and I'm trying to set up a design matrix to perform an ANOVA with 3 groups of subjects (controls, two groups of patients) and one covariate (task performance).
> >
> > I set up the design matrix like in the example below:
> >
> >
> > Group EV1 EV2 EV3 EV4
> > 1 1 0 0 9,70
> > 1 1 0 0 9,70
> > 1 1 0 0 9,70
> > 1 1 0 0 9,70
> > 1 0 1 0 7,70
> > 1 0 1 0 4,14
> > 1 0 1 0 -12,52
> > 1 0 0 1 -10,30
> > 1 0 0 1 -20,86
> > 1 0 0 1 -6,97
> >
> > CONTRASTS:
> >
> > 1 -1 0 0
> > 1 0 -1 0
> > 0 1 -1 0
> > -1 1 0 0
> > -1 0 1 0
> > 0 -1 1 0
> >
> >
> >
> >
> > Is this correct? Is it the same if I specify the value 1,2,3 for the column "Group" and split the performance column (EV4) in 2 different columns like below (demeaning for each column separately and deleting the column of the controls'performances, since it woul be all 0)?
> >
> >
> > Group EV1 EV2 EV3 EV4 EV5
> > 1 1 0 0 0 0
> > 1 1 0 0 0 0
> > 1 1 0 0 0 0
> > 1 1 0 0 0 0
> > 2 0 1 0 7,3 0
> > 2 0 1 0 3,75 0
> > 2 0 1 0 -12,9 0
> > 3 0 0 1 0 2.41
> > 3 0 0 1 0 -8.15
> > 3 0 0 1 0 5.74
> >
> >
> >
> > What is the difference between the two designs, and which is better?
> >
> >
> > Thank you in advance for your help.
> >
> >
> > Giulia