¥Multicollinearity Problem
_3dDeconvolve Error: Improper X matrix (cannot invert
XÕX)
_XÕX is
singular (not invertible) ↔ at least one column
of X is linearly dependent on the other columns
_normal equation has no unique solution
_Simple regression case:
"mistakenly provided at least two identical regressor
files, or some inclusive regressors, in 3dDeconvolve
"all regressiors have to be orthogonal (exclusive) with
each other
"easy to fix: use 1dplot to
diagnose
_Deconvolution case:
"mistakenly provided at least two identical stimulus
files, or some inclusive stimuli, in 3dDeconvolve
Ø easy to fix: use 1dplot to diagnose
"intrinsic problem of experiment design: lack of
randomness in the stimuli
Øvarying number of
lags may or may not help.
Ørunning RSFgen can
help to avoid this
_see AFNI_data1/ht03/bad_stim/c20.bad_stim