[Limdep Nlogit List] Exit Optimization without Status Code

[David Hensher]

Lognormals  have very long tails and this is a concern if you want to get WTP that are sensible, and often issue is data. If you want a specific sign then try a constrained normal or triangular. This does not solve the issue but experience suggests we resolve issue of very high or low estimates on the distribution. There are arbitrary analytical distributions and will always have issues and you need to be aware of this in trying different distns to try and explore an your data. This in art as much as science and one reason people use LCM

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On 24 Feb 2017, at 5:59 pm, Lixian Qian <Lixian.Qian at xjtlu.edu.cn<mailto:Lixian.Qian at xjtlu.edu.cn>> wrote:

Dear David,
Thank you for your prompt response.
Yes, we did transform the price variable by multiplying minus one and then ran the model with lognormal distribution assumption.
We are very intrigued by your comment that “this is quite common with lognormal”.
It will be highly appreciated if you can kindly expand on that comment in the context of using  different distributions for price in mixed logit model.
Thank you.

Best,
Lixian


From: David Hensher [mailto:david.hensher at sydney.edu.au]
Sent: 24 February 2017 14:49
To: Limdep and Nlogit Mailing List
Cc: Lixian Qian
Subject: Re: [Limdep Nlogit List] Exit Optimization without Status Code

This tells you that it cannot find a solution. Thts is quite common with lognormal.

A lognormal parameter is created as beta(i) = exp(b + sigma*w(i))
What you are looking at in your output are b and sigma.  Sigma has to be positive.  b can be positive or negative.
The lognormal distribution is used in RP models to constrain the sign of a coefficient. The
specification will appear as in ; Fcn = price(l). However, this forces the coefficient to
be positive. To force it to be negative the well known ‘trick’ is to multiply the price variable by
minus one, then force the coefficient to be positive as usual. Alternatively, the sign can be built into
the coefficient by using ; Fcn = -price(l).

David
On 24/02/2017 5:36 PM, Lixian Qian wrote:

Hello, we are running a mixed logit using NLogit.

The model runs well with coefficients with normal distributions (status =0), but if we change the distribution of monetary attributes to lognormal, the estimation ends with this statement.

"Line search at iteration   46 does not improve fn. Exiting optimization."



Can anyone please explain the issue? Thank you.





Best,

Lixian



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