[FieldTrip] interactions between two factors

[Marco Buiatti]

Thanks Arno and Stephen for your replies.

Stephen, the message you cite confirms that what I propose in my first
point is correct, thanks.

Any statistician willing to answer to my points 2) and 3) is welcome.

Best,

Marco

On 27 July 2012 16:25, Stephen Politzer-Ahles <politzerahless at gmail.com> wrote:
> 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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-- 
Marco Buiatti, PhD

CEA/DSV/I2BM / NeuroSpin
INSERM U992 - Cognitive Neuroimaging Unit
Bât 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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