[Limdep Nlogit List] start=logit with NL

[William Greene]

Kandice: Whether an MNL starting value is computed is independent
of the setting of RU1 vs. RU2. (At least it is according to the
internal program code.) You should not have to add ;Start=Logit
to the command. However, that will do no harm; the program should
always compute the initial MNL model. There is no other place to
get starting values. I can't say I can come up with a reason why
the MNL results were not reported.  On your ;IVSET instruction, 
you should use IVSET:(...)=[1] to force the value to equal one. I.e,
include it in square brackets. Otherwise it is just a starting value.
The program reports mu, not 1/mu.
/Bill Greene




----- Original Message -----
From: "Kandice L. Kleiber" <kleiberk at onid.orst.edu>
To: "Limdep and Nlogit Mailing List" <limdep at limdep.itls.usyd.edu.au>
Sent: Thursday, July 9, 2009 2:26:31 PM GMT -05:00 US/Canada Eastern
Subject: [Limdep Nlogit List] start=logit with NL

Hi -

I have a few questions regarding the *black box* in NLOGIT.  The  
nested logit model I have been running looks like this:
tree = cs(corn,soy), wh(wheat), ot(other), and the data does NOT very  
across alternatives - only across observations.

Because my nested model is a partially degenerate model, I'm using the  
RU1/RU2 commands to normalize.  When I use RU1, I add the restriction  
IVset:(wh,ot)=1.0.  However, when I run the 2 models, they "run"  
differently.  RU1 begins by running a MNL to get starting values,  
while RU2 does not, unless I call start=logit (at least not  
explicitly).  Even when I use the start=logit command on RU2, the  
value of my log likelihood function differs between RU1 and RU2 - from  
what I understand, this value should be the same (given the  
restriction with RU1).

Should I always include the start=logit command?  If not, when should I?
When the command RU2 is used, is the IV parameter NLOGIT reports = (1/ 
μ)?  Or does it report μ?

code:
nlogit;
shares;
tree = cs(corn,soy),wh(wheat),ot(other);
lhs = lev;
choices=corn,soy,wheat,other;
ru2;  /*or ru1 with restriction ivset:(wh,ot)=1.0*/
model:
U(corn) = c_c+c_price*price+ ... /
U(cs) = cs_cs+cs_soil*soil+.../
U(wheat)=w_w+w_soil*soil+...$

Any help is appreciated!  Thank you!
Kandice.

---
Kandice L. Kleiber
Graduate Teaching Assistant
Department of Economics
Oregon State University
e: kleiberk at onid.orst.edu
w: 541.737.6628

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