Hi, I'm afraid your colleague is right. The two approaches will produce
the same highest-level contrast value (if done right), but the second
approach has one degree of freedom per subject at the highest-level
analysis, which is correct, whereas your approach would generate twice
this number, so the second approach gives the correct varcope and degrees
of freedom at the highest level.
Cheers.
On Sat, 10 Sep 2005, Bradley Goodyear wrote:
> Basically, it's a scan where each subject views stimulus A (say for 5
> seconds) followed by crosshairs (say for 10 seconds) and stimulus B
> followed by crosshairs, repeated several times during a scan.
>
> > Hi Brad, I'm afraid I can't work out what you mean by "group of subjects
> > scanned under condition A and B is a randomized design within a scan" -
> > what exactly is going on at first level?
> >
> > Cheers, Steve.
> >
> >
> > On Fri, 9 Sep 2005, Brad Goodyear wrote:
> >
> > > Hi
> > >
> > > I was hoping that someone could settle an argument I am having with a
> > > colleague.
> > > Suppose I have a group of subjects scanned under condition A and B is a
> > > randomized design within a scan.
> > >
> > > I claim that it is best to generate COPEs for A and B separately for
> > > each individual at the first level, and then generate the A minus B
> > > contrast at the second level across the group.
> > > He claims it is best to generate the A minus B contrast for each
> > > individual at the first level, and then generate the group contrast at
> > > the second level.
> > >
> > > Which way is better?
> > >
> > > -BRad
> > >
> >
> > --
> > Stephen M. Smith DPhil
> > Associate Director, FMRIB and Analysis Research Coordinator
> >
> > Oxford University Centre for Functional MRI of the Brain
> > John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK
> > +44 (0) 1865 222726 (fax 222717)
> >
> > [log in to unmask] http://www.fmrib.ox.ac.uk/~steve
> >
>
--
Stephen M. Smith DPhil
Associate Director, FMRIB and Analysis Research Coordinator
Oxford University Centre for Functional MRI of the Brain
John Radcliffe 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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