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Yeah, that's what I thought. And basically that's why I asked in the first place :-) But thanks for all the contributions to this topic. I believe I have an idea on how to go about it, now. Best regards, Cornelius On Thu, Mar 31, 2011 at 5:06 PM, Michael Harms <[log in to unmask]> wrote: > 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 >> --------------------------------------------------------------------------- >> >> >> >> >> >> > -- Dr. med. Cornelius J. Werner Department of Neurology RWTH Aachen University Pauwelsstr. 30 52074 Aachen Germany