[Limdep Nlogit List] Latent class problems

[William Greene]

Dear Soren.
You should have constant terms in your model.
Add ;RH2=ONE to the command. That may not work, however, which
might suggest that the latent class model is not appropriate for
your data. On the results below. the 806 with 2 classes can be
taken as harmless if you have gone through a substantial number
of iterations - say more than 10. The results with 4 will be OK
if the suggested fix above gets you results with 3. Otherwise, 
you probably should not trust the 4 class results. Overall, the
singular covariance matrix suggests to me that at least one of
the classes is ending up with a trivial amount of mass, meaning
that there are too many classes.
B. Greene

----- Original Message -----
From: "Soren Olsen" <soeren_b_olsen at yahoo.com>
To: limdep at limdep.itls.usyd.edu.au
Sent: Monday, March 15, 2010 8:41:49 AM GMT -05:00 Colombia
Subject: [Limdep Nlogit List] Latent class problems

Dear all,

I'm experiencing some problems with a latent class model:

NLOGIT ;lhs=choose
;choices=1,2,3
;Rhs=catch_m,catch_h,size_m,size_h,nat_m,nat_h,qua_m,qua_h,dist_m,dist_h,num_m,num_h,p,optout
;LCM
;Prob=prob
;parameters
;maxit=200
;pds=6
;pts=2 $
 
My problem is that when I specify two classes as above, I get the following error message (though it does produce some results, I guess I shouldn't trust them):
Error 806: (The log likelihood is flat at the current estimates.)

When I use three classes (pts=3), I get another error message (and no results produced):
Error 1027: Models - estimated variance matrix of estimates is singular

However, when I use four classes (pts=4), it runs nicely and produces results that make very good intuitive sense, and everything looks ok.

With five classes (pts=5), I again get the following message:
Error 1027: Models - estimated variance matrix of estimates is singular
 
Now, I guess I actually have two questions:
1) Should I trust the results that I get with four classes? 
2) Why wont the other models run?

I should add that I have run RPL and EC models on exactly the same data and using the same utility function without any problems, so I guess my dataset is set up as it should be. I have tried specifying the utility functions using the model statement, but this didn't help

Any suggestions and help is much appreciated.

Regards
Søren

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