Dear SPMers, dear Rik,
I have used flexible factorial for a 2 (between subjects) by 2 (within
subjects) factorial design. Each group consisted of 4 subjects. This
resulted in the following design:
First 8 columns are subject's constants, last 4 columns regressors for
the repeated measures in the two groups.
1 0 0 0 0 0 0 0 1 0 0 0
1 0 0 0 0 0 0 0 0 1 0 0
0 1 0 0 0 0 0 0 1 0 0 0
0 1 0 0 0 0 0 0 0 1 0 0
0 0 1 0 0 0 0 0 1 0 0 0
0 0 1 0 0 0 0 0 0 1 0 0
0 0 0 1 0 0 0 0 1 0 0 0
0 0 0 1 0 0 0 0 0 1 0 0
0 0 0 0 1 0 0 0 0 0 1 0
0 0 0 0 1 0 0 0 0 0 0 1
0 0 0 0 0 1 0 0 0 0 1 0
0 0 0 0 0 1 0 0 0 0 0 1
0 0 0 0 0 0 1 0 0 0 1 0
0 0 0 0 0 0 1 0 0 0 0 1
0 0 0 0 0 0 0 1 0 0 1 0
0 0 0 0 0 0 0 1 0 0 0 1
The contrast for the comparison of groups is
0.5 0.5 0.5 0.5 -0.5 -0.5 -0.5 -0.5 1.0 1.0
-1.0 -1.0
Using an F test for this contrast gave an spmF image with negative F
values.
As the test compares only two groups I calculated also a T test for this
contrast. The resulting spmT image contained only zeros.
What was wrong? Using spm8b for the same design and tests gave reasonable
results.
BTW: The beta maps for the first 8 regressors (subjects) were slightly
different in spm5 and spm8b. The remaining 4 beta maps seem to be
indentical. What has been changed in the model estimation process?
Cheers and
thanks in advance
Bertram
--
Dr. Bertram Walter
Bender Institute of Neuroimaging
University of Giessen
Otto-Behaghel-Str. 10H
35394 Giessen
Germany
Phone +49 (641) 99-26307
or +49 (641) 99-26331 (Secretary)
Fax +49 (641) 99-26309
|