Dear Guillaume,
Thanks for you reply. I agree that SPM is not displaying anything wrong in the orthogonality matrix. But I suspect that the orthogonality matrix is just not meaningful (or even misleading) for investigating the efficiency of the design. Wouldn't the correlation matrix of the regressors (invariant of mean regressor values) be more helpful? The recource you cite stresses the importance of the correlations between regressors for an efficient contrast calculation (section "Correlation between regressors"):
http://imaging.mrc-cbu.cam.ac.uk/imaging/DesignEfficiency#head-525685650466f8a27531975efb2196bdc90fc419
I still have to check whether the efficiency (c'*inv(X'X)*c)^-1) is influenced by the mean of the regressors or not...
Thanks for your help, Adrian
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