Hi there
On Tue, 22 Jun 2004, Amande Pauls wrote:
> (1) Is it possible to control for an additional covariate that I am not
> interested in but suspect to have an influence on the outcome of the
> experiment (like measures of intelligence)? By modelling all of them as
> an additional EV? Or by making it one per subject (like when allowing
> for individual differences in variance)?
Yep - you should use a single EV for each covariate of no interest (e.g.
one EV for IQ, ine for age etc. etc. )
>
> (2) If I want to know whether there is a negative correlation between my
> additional covariate (modelled as a separate EV) and the data, do I need
> to set the contrast to -1?
>
Yes - absolutely right.
> (3) Having used an additional covariate (like the RT example in the
> webpages) and using that precise contrast, what does the result mean?
> I'm unclear on whether the brain area correlates both with RT and the
> task itself, or whether the level activity in that area somehow reflects
> RT, potentially independent of the task. Can I distinguish between those
> two cases, or make sure that my contrast reflects 'correlation with
> EV, given the task'?
>
It depends on the precise setup that you have chosen, but if you
orthogonalise such that RT explains the maximum possible variance (i.e.
orthogonalise all covariates of interest wrt RT) then the RT contrast
represents all the variance in the signal which _might possibly_ be
explained by RT. The copes of interest cannot then describe any variance
in the signal which could be ascribed to RT.
JHupw this is clear
Tim
> I hope this makes sense. Thanks for any help.
>
> Amande Pauls
>
> --------------------------------------------------------------------
> Amande Pauls
> University Laboratory of Physiology, Oxford, UK
> mailto:[log in to unmask]
>
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