Dear Julie,
> I am currently processing fMRI data that includes 2 sessions.
>
> Can anyone provide more info or a reference regarding Martin's statement to
> "Be aware when choosing the constrast because youŽll have both sessions in
> your design matrix."?
>
> For example, if I have 2 sessions and 3 conditions per session:
> 1: A B C
> 2: A B C
>
> If I wanted to define the contrast A -B, would the contrast weights be:
>
> 1 -1 0 1 -1 0
>
> or
>
> 0.5 -0.5 0 0.5 -0.5 0
These two contrasts will give you equivalent t statistics, but the
contrast estimates will be different (though the relative size is the
same), because the error scales with the contrast weights. The second
one - where the positive and negative contrast weights sum to 1 - is
technically the average of the conditions. But because the statistics
come out the same, I don't think people usually get too bothered about
it.
Hope this helps!
Best regards,
Jonathan
--
Dr. Jonathan Peelle
Center for Cognitive Neuroscience and
Department of Neurology
University of Pennsylvania
3 West Gates
3400 Spruce Street
Philadelphia, PA 19104
USA
http://jonathanpeelle.net/
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