Dear SPM experts
we try to run a F-test with 4 groups of subjects to find differences between any of the 4 groups and have recognized that there are different recommendations with respect to the contrast weights.
Some people suggest to use eye(n)-1/n, i.e.
0.7500 -0.2500 -0.2500 -0.2500
-0.2500 0.7500 -0.2500 -0.2500
-0.2500 -0.2500 0.7500 -0.2500
-0.2500 -0.2500 -0.2500 0.7500
Other people suggest to use the contrast
1 -1 0 0
0 1 -1 0
0 0 1 -1
Results are different and therefore we would like to know which contrast is more appropriate (or even wrong) to find differences between any of the 4 groups.
Thanks in advance for your advice
Best regards
Jürgen
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