Dear SPM'ers
I have been looking at how similar columns in a multiple regression
model need to be, in order not to be estimable.
The design matrix i have looked at is: X=[c1 c2 c3 mu]
c1 and c2 were regressors with std=1 and mean=0, c3=c2+e and i have then
looked at the degrees of freedom as a function of "e". When "e" gets
below ~ 10^-13 the columns c2 and c3 becomes inestimable, and the number
of freedoms increases with one.
That is a bit late i think specially because multiple regression could
be used for designs having low degrees of freedom. Wouldn't one expect
that the degrees of freedom available for estimating the contrast
belonging to c1 should be the same no matter if e=0 or e=10^-13 ?
Torben Ellegaard Lund, M.Sc.
Danish Research Centre for Magnetic Resonance
|