Sorry I tried to avoid "A" and "B" so I mixed up my notation.
Again (and please disregard my 1st posting)
Hi,
now I am confused because I think Karli is right.
On Saturday 12 November 2005 15:40, Alle Meije Wink wrote:
> Dear Karli,
>
> >I did an fMRI study that involved a group of subjects doing a task that
> >that varied along factors A and B and along factors 1 and 2, and I'm
> >interested in the interaction effect. To address this, I ran individual
> >t-tests of the type [(A1-A2)-(B1-B2)] and [(B1-B2)-(A1-A2)] and
> >then did a second level random effects analysis.
> >However, the person reviewing my paper is insisting that I do an ANOVA,
> >and claims that the t-tests I ran are not true interaction maps, and that
> >they will identify brain regions showing two main effects and no
> >interaction. Here is his/her example:
> >
I hope I read this correctly as 2 Factors (let's call them F1, F2) with 2
levels each (F1(1) F1(2) F2(1) F2(2)) and you build contrasts
[F1(1)F2(1)-F1(2)F2(1)]-[F1(1)F2(2)-F1(2)F2(2)]?
(A=F1(1),B=F1(2),1=F2(1),2=F2(2))
Why don't you take the F contrast over that contrast instead of 2 T-contrasts
(the "direction" of an interaction term is pretty hard to interpret, isn't
it?). But other than that I don't see the incorrectness.
> So the maps you made are
> 1. [(A1-A2)-(B1-B2)] = main effect AB + main effect 12 together in a map
> 2. [(B1-B2)-(A1-A2)] = main effect BA + main effect 12 together in a map
This is interaction: If the difference between cond1 and cond2 is different in
condA then condB, the maineffect(1-2) depends on the other effect (A-B). This
is exactly the 3rd coloumn in fig 7 of Rick Hanson's excellent technical
guide to Anova.
http://www.fil.ion.ucl.ac.uk/~wpenny/publications/rik_anova.pdf
> I don't think you can conclude anything about interactions from those.
> What you need is a repeated measures ANOVA, set up in the following way:
Like I said, I think he's right. Can someone enlighten us?
Regards,
Roland
--
Dr. Roland Marcus Rutschmann <[log in to unmask]>
Institute for Experimental Psychology, University of Regensburg
Universitätsstraße 31, 93053 Regensburg, Germany
Tel: +49 941 943 2533, Fax: +49 941 943 3233
http://www.psychologie.uni-regensburg.de/Rutschmann
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