Just wanted to chime in that demeaning the performance EV separately
within group is a rather unique case that is specific to this particular
post.
Recent posts by Jesper (just yesterday), Jeannette, Tom, and myself have
all advised that, in general, one should demean across all subjects (NOT
within group separately).
Given the recent posts on this, I thought it was worth making explicit
that demeaning within groups is not a "typical" situation.
And as a matter of good reporting practice, any time that demeaning is
performed separately within group, rather than across all subjects, that
should be noted (and justified) very explicitly in any presentation of
the ensuing results.
cheers,
-MH
On Thu, 2011-03-31 at 08:42 +0100, Stephen Smith wrote:
> Hi
>
> On 30 Mar 2011, at 11:30, Cornelius Werner wrote:
>
> > Hi,
> >
> > sorry to revive such a well-worn topic. But there is something I did
> > not quite get so far.
> > As an example, I am examining a patient cohort and a control cohort
> > in
> > a Dual Regression setup (resting state data). Patients and controls
> > are matched for age and gender. They obviously differ in diagnosis,
> > but also in one performance score. I am interested in basic group
> > differences and the differential correlation of connectivity
> > strength
> > of several RSNs with performance. For the final randomise-step, my
> > design matrix has a column for group mean "patient" and one for
> > "controls" (consisting of 1, padded with zeroes where applicable),
> > and
> > two separate columns for age (as a confounder) - one for each group,
> > respectively, because an age*group interaction on connectivities
> > could
> > not be excluded a priori. As I was modelling the group mean
> > separately, only the slopes associated with age were tested. Is that
> > correct so far?
>
>
> I think so - sounds fine.
>
> > As the age means did not differ (tested beforehand),
> > does it matter if I demeaned within group or across groups?
> > Shouldn't
> > the intercept be modelled by the group mean regressor, in any case?
> > Following Tom's last post, I'd probably demean across groups.
> >
> > The next thing is even more unclear to me:
> > Due to an expected group*performance interaction (i.e. steeper slope
> > of increases in connectivity along with better performance in
> > contrast
> > to the other group), also the performance scores are split. BUT:
> > should I demean?
>
>
> Yes - if you want to compare the *slopes* between the two groups,
> demean the performance scores within group before padding with zeros,
> for each group's performance EV.
>
> > And if so, within groups, or across groups? In this
> > case, mean differences in performance are believed to be *due to*
> > diagnosis - therefore, variability associated with the mean should
> > go
> > to the group regressor, shouldn't it? In this case, I'd be inclined
> > to
> > demean in order not to affect the group mean regressor negatively,
> > and
> > to demean within groups, because of the (clearly) attributable mean
> > variability...?!
> >
> > Example:
> >
> > EV1: Patient mean
> > EV2: Control mean
> > EV3: Patient age (demeaned across groups - EV of no interest)
> >
>
>
> I presume you mean demeaned within group, then padded with zeros.
>
>
> Cheers.
>
> > EV4: Control age ( " )
> > EV5: Patient performance score (demeaned within patients)
> > EV6: Control performance score (demeaned within controls)
> >
> > Patients>controls: 1 -1 0 0 0 0
> > Controls>patients: -1 1 0 0 0 0
> > Slope(performance score) patients > Slope(performance score)
> > controls:
> > 0 0 0 0 1 -1
> > Slope(performance score) controls > Slope(performance score)
> > patients:
> > 0 0 0 0 -1 1
> >
> > Please don't hit me - I'm having a hard time getting my head around
> > this :-)
> > Cheers,
> > Cornelius
> >
> >
>
> ---------------------------------------------------------------------------
> Stephen M. Smith, Professor of Biomedical Engineering
> Associate Director, Oxford University FMRIB Centre
>
> FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
> +44 (0) 1865 222726 (fax 222717)
> [log in to unmask] http://www.fmrib.ox.ac.uk/~steve
> ---------------------------------------------------------------------------
>
>
>
>
>
>
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