swann
> Thanks a lot, the tutorial is indeed very clear and helpful.
> But I admit I still have difficulties to understand the gain of
> including main effects and interactions as regressors of the GLM (eg
> design3 page 10) additionally to the different levels of the condition
> factor, as compared with estimating main effects and interactions
> using contrasts (eg simplest design1 page 8). Page 11, Glascher and
> Gitelman explain that including main effects in the design matrix
> helps gaining sensitivity for detecting an interaction.
> But doesn't it cost a lot in terms of DOF ?
in the classical ANOVAs, main effects and interaction are estimated with
a pooling of the Ys per factors for main effects ie the variance
partition is different between models - when choosing designs I tend to
use only main subjects and interations between factors but it's clearly
a trade off - one the one hand, including main effects reduces the error
terms and therefore helps to detect interactions (as only looks at
variance not explained by mains); on the other hand (see end of the
tutorial) the roughness of the residual increases and therefore
decreases the significance .. there is not definitive answer here ..
hope this helps
Cyril
>
> Best wishes
>
>
> --
> Swann Pichon, PhD
> Laboratory for Behavioral Neurology and Imaging of Cognition
> Department of Neuroscience, University Medical Center
> 1 rue Michel-Servet, 1211 GENEVA 4, Switzerland
> Pho: +41 (0)22 379 5979
> Fax: +41 (0)22 379 5402
> Gsm: +33 (0)6 26 43 83 61
> http://labnic.unige.ch/
>
>
> 2010/1/6 Richard Morris <[log in to unmask]
> <mailto:[log in to unmask]>>
>
> I've found this the most helpful document so far on running ANOVAs
> in SPM:
>
> http://www.sbirc.ed.ac.uk/cyril/downloads/Contrast_Weighting_Glascher_Gitelman_2008.pdf
>
>
> Cheerio,
>
> Rich.
>
>
> On 05/01/2010, at 2:38 AM, Vaidya, Jatin G wrote:
>
> > Thanks! I know I can run this as a between group t-test after
> doing the
> > subtraction but I'd like to know how to do it as an ANOVA.
> >
> > The link you sent was helpful. I'm curious if there is any other
> > information available on how to actually implement this in SPM
> (more of
> > an applied example).
> >
> > Thanks for your help.
> >
> > Jatin
> >
> > -----Original Message-----
> > From: Stephen J. Fromm [mailto:[log in to unmask]
> <mailto:[log in to unmask]>]
> > Sent: Friday, January 01, 2010 7:53 AM
> > To: [log in to unmask] <mailto:[log in to unmask]>; Vaidya, Jatin G
> > Subject: Re: 2 x 2 ANOVA
> >
> > On Thu, 31 Dec 2009 22:38:19 +0000, Jatin Vaidya <jatin-
> > [log in to unmask] <mailto:[log in to unmask]>> wrote:
> >
> >> Hi,
> >>
> >> I'm trying to run a 2 X 2 design with PET data. The first
> factor is
> > group
> > (user
> >> vs. control). The second factor is task (tapping vs. control);
> thus,
> > this is a
> >> repeated measures variable.
> >>
> >> I can't seem to determine how to run the analysis in SPM with
> this type
> > of
> >> dataset. Is there a good resource available for this?
> >
> > http://www.fil.ion.ucl.ac.uk/~wpenny/publications/rik_anova.pdf
> <http://www.fil.ion.ucl.ac.uk/%7Ewpenny/publications/rik_anova.pdf>
> >
> > In this particular case, if you're only interested in comparing
> tapping
> > with
> > control (i.e., not interested in tapping alone or control alone
> or their
> > average,
> > but only their difference), you could also just subtract control
> from
> > tapping at
> > the subject level (or using ImCalc), and bring the difference to the
> > group
> > level, resulting in a 1-way, between-subjects ANOVA with no repeated
> > measures.
> >
> >>
> >> Thanks,
> >>
> >> Jatin
>
>
>
> --
> Swann Pichon, PhD
> Laboratory for Behavioral Neurology and Imaging of Cognition
> Department of Neuroscience, University Medical Center
> 1 rue Michel-Servet, 1211 GENEVA 4, Switzerland
> Tel: +41 (0)22 379 5979
> Fax: +41 (0)22 379 5402
> Gsm: +33 (0)6 26 43 83 61
> http://labnic.unige.ch/
>
--
Dr Cyril Pernet,
fMRI Lead Researcher SINAPSE
SFC Brain Imaging Research Center
Division of Clinical Neurosciences
University of Edinburgh
Western General Hospital
Crewe Road
Edinburgh
EH4 2XU
Scotland, UK
[log in to unmask]
tel: +44(0)1315373661
http://www.sbirc.ed.ac.uk/cyril
http://www.sinapse.ac.uk/
The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.
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