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st: nlogit defiant dissimilarity parameters


From   "Filippo Maria D'Arcangelo" <filippo.darcangelo@unibocconi.it>
To   statalist@hsphsun2.harvard.edu
Subject   st: nlogit defiant dissimilarity parameters
Date   Thu, 20 Jun 2013 15:17:22 +0200 (CEST)

Dear STATA users,

I'm quite new to the nested logit model and I'm not sure to have it completely clear in its every aspects.
In particular, I tried up the example in the nlogit help (you can find the commands hereafter):

        . webuse restaurant
        . nlogitgen type = restaurant(fast: Freebirds | MamasPizza, family:  CafeEccell | LosNortenos | WingsNmore, fancy: Christophers | MadCows)
        . nlogit chosen cost distance rating || type: income kids, base(family) || restaurant:, noconst case(family_id)

The output shows, among others, these results for IV or "dissimilarity parameters":

> ------------------------------------------------------------------------------
> dissimilarity parameters
> ------------------------------------------------------------------------------
> type         |
>    /fast_tau |   1.712878    1.48685                     -1.201295    4.627051
>  /family_tau |   2.505113   .9646351                       .614463    4.395763
>   /fancy_tau |   4.099844   2.810123                     -1.407896    9.607583
> ------------------------------------------------------------------------------
> LR test for IIA (tau = 1):           chi2(3) =     6.87   Prob > chi2 = 0.0762
> ------------------------------------------------------------------------------


As far as i know they should be comprised between 0 and 1 for the model to be coherent with RUM. Am I right here or am I mistakenly interpreting them?
It seems like the model is not normalized (i have read that this was the case of nlogit before STATA 10).
I even tried to launch the nlogitrum command. The results differ, without a clear explanation to me, while the IVs keep on being still bigger than 1.

What do I miss to understand here?
Thank you in advance for your kind reply.

f.
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