Hi,
in addition to what has been said let me just point out that having two "sessions" or "runs" in general refers to the fact that the scanning has been stopped, i.e. that you have two separate time series. In that regards, it does not matter if the 2nd scan was obtained on the same or another day. Often, people tend two treat a set of 2 time series as 2 different sessions even if the subject remained in the scanner and the scanning was just halted (even though in that case you may sometimes just concatenate the sessions into a single 4D data set after discarding the volumes that were not yet in steady state.)
Cheers-
Andreas
________________________________
Von: FSL - FMRIB's Software Library im Auftrag von Tim Behrens
Gesendet: Mi 27.07.2005 20:04
An: [log in to unmask]
Betreff: Re: [FSL] Third Level Aalysis - Fixed or Mixed?
If you want to explicitly include, and estimate, a between run variance,
then you should do mixed at the second level, (but you may have to assume
that the between run variance is the same for all subjects - it depends on
the number of data runs). Otherwise, if you are happy for the between run
variance to be soaked up in the between subject variance, you can just do
fixed at the second-level. This won't affect the generalisability of the
cross subject stats.
Hope this helps
T
On Wed, 27 Jul 2005, Brad Goodyear wrote:
> Thanks Tim (and Mark and Steve).
>
> in the second level of analysis for repeated measures, if the
> "sessions" are conducted within the same imaging scanning session (hope
> that makes sense) rather than on different days, would this be a
> candidate for fixed effects at the second level, or does it still come
> down to generalizability?
>
> -Brad
>
> On Jul 27, 2005, at 1:57 AM, Tim Behrens wrote:
>
> > Brad
> >
> > At the final level, The rule of thumb is:
> > If I am only interested in these exact people in my group, use fixed.
> > If I want to find out something about the population that these people
> > come from, use mixed.
> >
> > This rule of thumb can be simplified slightly to the following:
> > Use mixed.
> >
> > At earlier levels, it is also generally better to use mixed (stage 1 -
> > for
> > speed), but there are some examples where you can use fixed if you
> > want to
> > make certain specific assumptions about particular variance components.
> >
> > Cheers
> >
> > T
> >
> >
> >
> > On Wed, 27 Jul 2005, Bradley Goodyear wrote:
> >
> >> After using FLAME Mixed Effects Stage 1 only in the repeated measures
> >> example, is the group mean a fixed or mixed effects model? IS there a
> >> rule of thumb for using fixed versus mixed?
> >>
> >> -BRad
> >>
> >
> > --
> > -----------------------------------------------------------------------
> > --------
> > Tim Behrens
> > Centre for Functional MRI of the Brain
> > The John Radcliffe Hospital
> > Headley Way Oxford OX3 9DU
> > Oxford University
> > Work 01865 222782
> > Mobile 07980 884537
> > -----------------------------------------------------------------------
> > --------
>
--
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Tim Behrens
Centre for Functional MRI of the Brain
The John Radcliffe Hospital
Headley Way Oxford OX3 9DU
Oxford University
Work 01865 222782
Mobile 07980 884537
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