Hi,
I see your point, I will drop the performance regressor at this level of
the analysis. Thank you for helping me sort this out.
Hanne
Sitat Steve Smith <[log in to unmask]>:
> Hi,
>
> If you have two regressors that are the same then the fitting
> mathematics will be poorly conditioned and you will not be able to
> run
> such contrasts as:
> [1 0]
> [1 -1]
> [0 1].
>
> In this case you will have to drop the performace EV I suspect.
> However, if the subject performs exacrtly the same for every task it
> doesn't make sense to add it anyway. Maybe instead you maybe want to
> be adding a performance regressor at the group level, in which case
> you wouldn't need to have it like the in the first level.
>
> Cheers, Steve.
>
>
>
> On 24 Jan 2008, at 14:32, Hanne Lehn wrote:
>
> > Hi again,
> >
> > I have now run the first level analyses discussed in previous
> e-mails,
> > with an added regressor that models performance effects. For some
> > subjects, I get a warning about my design being rank deficient. It
> > seems
> > it occurs whenever the performance regressor (EV2) is (too close to
> > being)
> > colinear to the main regressor (EV1).
> >
> > Example: The onsets and durations of EV1 and EV2 are always
> > identical, as
> > they model the same event; EV1=main effect, EV2= performance
> > modulation.
> > EV1 is always weighted with 1's (3rd column of onset file)
> Sometimes
> > EV2
> > is also weighted with only 1's (if performance equaled 1 point on
> > every
> > trial), or perhaps only 2's. In these cases my design is regarded
> rank
> > deficient, and the resulting statistics don't make any sense (EV2
> z-
> > scores
> >> 50).
> >
> > Does anyone know how to deal with this problem?
> >
> > From the past correspondence I understand there is no need to
> demean
> > the
> > performance scores before entering them as weights for EV2, as long
> > as EV2
> > is orthogonalized wrt EV1. What, then, could I do to get a proper
> > design?
> >
> > Thanks a lot in advance!
> >
> > Hanne
> >
>
>
>
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> 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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>
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