Just want to share some knowledge and question at the same time how this can be.
In a large design, where subjects came for fMRI in 2 sessions, I want to enter a physiological measure as covariate.
Created an extra EV and entered for each subject the demeaned value of the measure twice (one for each session).
The design was rejected as "rank deficient".
The graphical representation of the design matrix and parameter contrasts showed that the "red line" for some subjects/values was extremely to the right or left position, indicating a relatively large difference from 0 zero.
there were indeed high values >10 and some >100 here.
I decided to divide all values /100 and entered these values in a new design (demeaned/100 = still demeaned, I reasoned).
Now it was accepted ! without rank deficiency messages.
1) If this is correct: then it can be of interest for all users, that these demeaned values for covariates should not exceed certain numbers.
2) I would like to know if there is any argument against this procedure.
Drs. Anda van Stegeren,
University of Amsterdam
Department of Clinical Psychology
1018 WB Amsterdam
tel. ++ 31 20 525 6799
fax. ++31 20 639 1369
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