Thanks a million!
On Thu, Mar 31, 2011 at 9:42 AM, Stephen Smith <[log in to unmask]> 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
> ---------------------------------------------------------------------------
>
>
>
>
--
Dr. med. Cornelius J. Werner
Department of Neurology
RWTH Aachen University
Pauwelsstr. 30
52074 Aachen
Germany
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