Depending on the specific contrasts you are interested in, demeaning may
or may not impact those contrasts, as the webpage explains. In general,
demeaning is "safer".
cheers,
-MH
On Fri, 2011-05-20 at 04:01 -0700, R Duke wrote:
> Cheers Michael. Demeaning across all subjects makes sense. However, it
> also looks like I should enter RAW values in for my covariates of NO
> interest. Is that correct?
>
>
> Young Old Cov_interest_young Cov_interest_old
> Cov1 Cov2
> 1 0 20
> 0 100 500
> 1 0 44
> 0 200 560
> 0 1 0
> 33 160 450
> 0 1 0
> 45 222 720
>
>
>
>
>
>
>
>
>
>
> ______________________________________________________________________
> From: Michael Harms <[log in to unmask]>
> To: [log in to unmask]
> Sent: Thu, May 19, 2011 5:16:31 PM
> Subject: Re: [FSL] Randomise Interaction
>
> See the following web page, which gives helpful examples on demeaning,
> and answers your Q3 (A: demean across all subjects)
>
> http://mumford.fmripower.org/mean_centering/
>
> On Thu, 2011-05-19 at 21:31 +0100, R. Duke wrote:
> > Hello Experts,
> >
> > I have tried my best to confirm my design with the postings, but I
> feel like I need to confirm with someone on the listserv. Here is my
> design.
> >
> > 2 groups (young and old)
> > 2 nuisance covariates (cov1, cov2)
> > 1 covariate of interest (cov_interest)
> >
> > I want to test whether there is a groupXcov_interest interaction
> (i.e., differential slopes between young and old in cov_interest).
> >
> > Young Old Cov_interest_young Cov_interest_old
> Cov1 Cov2
> > 1 0 20
> 0 100 500
> > 1 0 44
> 0 200 560
> > 0 1 0
> 33 160 450
> > 0 1 0
> 45 222 720
> >
> >
> > Contrast1 = 0 0 1 -1 0 0
> > Contrast2 = 0 0 -1 1 0 0
> >
> > Questions:
> > 1. Does this design/constrast(s) appear correct?
> > 2. Should I demean any of my covariates or should I just enter
> RAW values into each of my four columns?
> > 3. If I should demean any of my covariates, should I demean
> within or between my two groups? This one seems to be highly debated.
> >
> >
> > Cheers!
>
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