I conducted two analyses that I thought should give identical results (but
apparently not). It is a six subject, two condition (with one replication)
study (BABA, where B=baseline and A=activation, in that order).
I used two designs:
1. Multisub, Cond x Subj & Covar.
Given the subject by condition interactions, the contrast for the entire
effect across all subjects was modeled as: [-1 1 -1 1 -1 1 -1 1 -1 1 -1 1]
(i.e., one contrast for each subject, comparing the activation to the
basleine conditions).
2. Multisub, Cond & Covar:
Collapsing across subjects, the contrast was simply [-1 1].
Although the results are quite similar, they are not identical. In
particular, a theoretically meaningful area emerges in the first design
(suprathreshold), but not in the second (sigificantly subthreshold, but
still there).
Can someone tell me why? My first guess is that the variance terms used may
be different for each. Also, is it inappropriate to use the first (i.e.,
perhaps too many comparisons in a single model?)?
________________________________
David A. Kareken, Ph.D., ABPP
Assistant Professor & Director
Neuropsychology Section, Department of Neurology
RI 5999C
Indiana University School of Medicine
Indianapolis, IN 46202
(317) 274-7327 (voice)
(317) 274-1337 (fax)
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