[FieldTrip] interactions between two factors

[Stephen Politzer-Ahles]

Hello Marco,

For how to test an interaction in a 2x2 design, see
http://mailman.science.ru.nl/pipermail/fieldtrip/2011-January/003447.html(and
some additional information at
http://mailman.science.ru.nl/pipermail/fieldtrip/2010-December/003338.html,
http://mailman.science.ru.nl/pipermail/fieldtrip/2011-September/004244.html,
and Anderson & Braak 2003 in *Journal of Statistical Computation &
Simulation*).  I'm not sure about the other two issues you mentioned,
though.

Best,
Steve

Message: 1
> Date: Thu, 26 Jul 2012 12:36:39 +0200
> From: Marco Buiatti <marco.buiatti at gmail.com>
> To: fieldtrip at donders.ru.nl
> Subject: Re: [FieldTrip] interactions between two factors
> Message-ID:
>         <
> CA+wnTO2TL7XtzsKX2QciqXZO9hVxGUZTPjXwoYtS54ztRc0KeQ at mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
>
> Dear FieldTrippers,
>
> some time ago I have posted the message below concerning how to
> compute the statistical interaction between two factors in an EEG
> study with the FieldTrip cluster-based statistical analysis. Since I
> believe it is a problem of general interest, I was confident I would
> have received some replies, no matter how critics. But I had no reply
> and I am trying to guess why:
>
> - the problem is trivial, I should go back to my statistics books and
> solve it myself;
> - the problem is ill-posed, I should go back to my statistics books
> and reformulate it correctly;
> - the problem is tabou, no one dares commits to a solution because it
> could be a wrong one.
> - the problem is solved: I should read message number #.
>
> Thanks a lot for your feedback,
>
> Best,
>
> Marco
>
> On 25 May 2012 15:58, Marco Buiatti <marco.buiatti at gmail.com> wrote:
> > Dear FieldTrippers,
> >
> > I am analysing an EEG study with 2x4 factors: one varies between 4
> > parametrically varying levels (1 to 4), the second between two levels.
> >
> > I have three questions concerning the use of Fieldtrip cluster-based
> > non parametric statistical analysis in this case:
> >
> > 1) How to compute the interaction between the two factors. Let's start
> > from the simplest case of a 2x2 design, factors varying between values
> > A1 and A2 for the first factor, B1 and B2 for the second. Please tell
> > me if it is correct to compute the interaction by:
> > - computing the difference diffA=ERP(A1)-ERP(A2) separately in
> > condition B1 and B2, for every subject
> > - performing a within-subjects statistical analysis between diffA in
> > condition B1 and diffA in condition B2 (function
> > statfun_depsamplesT.m).
> >
> > 2) Now consider that factor A varies parametrically between values 1
> > to 4. For the main effect of this factor, I have used the Fieldtrip
> > function statfun_depsamplesregrT.m and I'm satisfied with it. Is it
> > correct to compute the interaction by
> > - computing the regression
> > regrA=regression(ERP(A1),ERP(A2),ERP(A3),ERP(A4)) (computed as inside
> > function statfun_depsamplesregrT.m) separately in condition B1 and B2,
> > for every subject
> > - performing a within-subjects statistical analysis between regrA in
> > condition B1 and regrA in condition B2 (function
> > statfun_depsamplesT.m)?
> >
> > 3) Since BEFORE looking at the data (this is to prevent Eric's
> > contestation...) I expect a dipolar topography for the regression
> > (data are in average reference), I would like to combine into a joint
> > cluster negative and positive clusters. I have tried by changing
> > statfun_depsamplesregrT.m by just taking the absolute value of the
> > regression, but I get weird results (a huge, non significant cluster).
> > Is it possible that since values are now all positive, I should use a
> > different statistical test at the single bin level? Any other
> > suggestions?
> >
> > Thanks in advance for your help,
> >
> > Marco
> >
> >
> >
> > --
> > Marco Buiatti, PhD
> >
> > CEA/DSV/I2BM / NeuroSpin
> > INSERM U992 - Cognitive Neuroimaging Unit
> > B?t 145 - Point Courrier 156
> > Gif sur Yvette F-91191  FRANCE
> > Ph:  +33(0)169.08.65.21
> > Fax: +33(0)169.08.79.73
> > E-mail: marco.buiatti at gmail.com
> > http://www.unicog.org/pm/pmwiki.php/Main/MarcoBuiatti
> >
>
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