[Limdep Nlogit List] Sample Selection Model - Error 806

Shane Murray shane.murray at memetrics.com
Tue Aug 7 18:47:21 EST 2007


I'm attempting to run a sample selection model on a continuous dependent
variable with a high degree of censoring (approximately 88% zeros). 

Commands as follows:
Namelist	;x=one,x12,x13,x14,x15,x16,x22,x31,x33,x41,x42,x43,x52 $
Probit	;lhs=YGT0 ;rhs=x ;hold $
selection	;lhs=LOGY ;rhs=x ;mle; maxit=1000 $

Note that my independent variables are nominal variables, which I have
dummy coded.

I have taken a couple of random slices of the data (the dataset has
approx 600,000 records), and when I run the model I am seeing very
different results for each slice.

In particular, RHO can vary from +1 to -1.

The following message is also presented:
"Line search does not improve fn. Exit iterations. Status=3
Check derivatives (with ;OUTPUT=3). This may be a solution
if several iterations have been computed, not if only one.
Error   806: (The log likelihood is flat at the current estimates.)"

What does it mean by "the log likelihood is flat at the current
estimates"?
Is there something I can do to arrive at a more stable solution?


Any assistance is greatly appreciated.

Thanks,
Shane




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