Could someone provide a simple explanation of the following design options
for a
PET study. My study is an PET-FDG study with 2 scans per subject, with 1
performance covariate collected per scan.
1) Condition X subject interaction and covariate - What is the advantage of
this
design ? I get an error message "unestimable condition effects"
2) Centering options - what are the relative merits and conditions where one
would use overall centering of the co-variate, subject centering, and
condition
centering. Which is the same as the procedure recomended by Andrew Holmes
for
using difference covariate scores in SPM96 (mean center the difference
scores,
divide by 2, alternately multiply by -1 and 1 for baseline and activation
scans).
Thanks.
sg
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