Hi,
I'm a 'greenhorn' concerning SPM and have a few questions. I'd be
very happy, if someone has some suggestions.
The problem is the following: We have a PET study with two groups, each
of them with two conditions (alertness and some kind of GonoGo-Test),
6 Scans per subject and condition. Furthermore there is a
neuropsychological score for the GonoGo-Test, which shouls be used as a
covariate. Ok, after preprocessing the data I calculated the mean of the
realigned, smoothed and normalized PET Scans. With these I used the
multi-study, conditions and covariates design. Now the questions
1.) I compared the results from the mean data with those obtained
without avering and using multi-study with repititions design for
a simple contrast. The results were significantly different.
Is there an explanation ?
What is the better design then ????
2.) One interesting thing is the interaction of the covariates
with a simple contrast, let's say (GonoGo-alertness). Are the contrasts
(0 0 -1 1 1) and (0 0 -1 1 -1) the right ones ? The third and fourth
number are alertness and GonoGo for one of the two groups. And is
(-1 1 -1 1 1) describing the overall effect ????
3.) What is the form, in which the covariates have to be given ?
I have read in the mailbase, that they should be positive and negative??
Is that right???
Thanks
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