Hi all,
In an attempt to work through the statistics in SPM, I stumbled
upon some issues I do not understand.
1. The factors in the design matrix are centered around 0. Why is
that? Wouldn't it be better if this were not the case?
2. What contrast is used when one wants to see activation related
to one factor alone? Are these values compared to the grand mean
of the other factors? Or to activation from the rest condition?
3. I found that two of the 4 factors in our design were correlated
(r = -.69). I assume that this is not desirable. But what is the
impact of such a correlation between two factors which both are
expected to show activation in the motor areas?
Thanks in advance,
Matthijs
Matthijs Vink
Functional Imaging
Dept. Psychiatry
University Medical Center
Room A.01.126
Heidelberglaan 100
3584 CX Utrecht
The Netherlands
tel. (31) 30 250 7121 (work)
tel. (31) 30 250 8352 (work)
tel. (31) 30 231 0171 (home)
www.fMRI.nl
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