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Re: st: asmprobit, problems with correlation patterns


From   "Barbara Hanel " <[email protected]>
To   [email protected]
Subject   Re: st: asmprobit, problems with correlation patterns
Date   Mon, 23 Apr 2007 17:53:54 +0200

Hallo Rich,
In the first run I tried to estimate the model just with the a-s- Variable "eink" - it doesn�t work too. In fact the model converged, but estimated a correlation coefficient of exactly -1. Furthermore, I got extremly large standard errors for the variance parameter. (It was a specification with only 3 destination states, so there was only one covariance parameter beside the variance parameter.)
I�t seems to me to be a similar problem like in the model with 4 destination states and a correlation coefficient of _exactly_ -0.5 But I just don�t know what it in fact could be.
That problem even persists only with the as-variable.

I defined the restricted pattern primarily because I hoped it would improve the estimation compared to the full model, because there are less parameters to identify. So I have even less hope to find a solution with the default option.



I hope I don�t waste too much of your time...

Thanks a lot,
Barbara


Am 23 Apr 2007, um 8:09 hat Richard Gates geschrieben:

Barbara,

If the correlation problem persists after dropping the
c-s variable from the indepvar variable list let me
know.

Try the default differenced covariance, it should give
you the optimal likelihood. It will produce a 3x3
covariance, estimating 5 Cholesky transformed covariance
parameters. These estimates are not interpretable, but
generally the covariance parameters are considered a nuisance
parameters. It will also give you the full model to compare
with your restricted (in the covariance structure) model using
-lrtest-

I believe it is the alternative-specific (a-s) variables that
identify the covariance parameters since they vary within
each case. I have tried running models with and without a-s
variables using the differenced covariance structure and as
soon as I drop the a-s variables -asmprobit- does not converge.
So I think your eink variable will be key in identifying the
covariance parameters.

My guess is that once you drop AB from indepvars estimation
will improve.

Best Regards,

-Rich

On Sun, 2007-04-22 at 15:39 +0200, Barbara Hanel wrote:
> Hallo,
>
> That sounds good. (I thought I first had to define all explanatories
> and than wich of them is case-specific.)
> I will try as you suggested.
>
> Of course I would still be appreciated if there is anybody with an
> idea concerning the correlation-pattern problem...
>
> Thanks a lot, Rich!
> Barbara
>
> Am 20 Apr 2007, um 12:48 hat Richard Gates geschrieben:
>
> Barbara inquired about the collinear message she is receiving
> from -asmprobit-:
>
> > 2) If I include more than one explanatory variable in the model I get
> > the message:
> > "Note: two or more of the variables are collinear; convergence may
> > not be achieved"
> > That's the case even if I include:
> > * one variable varying across alternatives and cases and
> > * only one other variable that is constant across the alternatives,
> > varies only by "i".
> >
> > I don´t understand how two such variables could be collinear?
>
> I will include a snippet of her output below
>
>
> > ------------------------------------------------------------------------------
> > wahl | Coef. Std. Err. z P>|z| [95% Conf. Interval]
> > -------------+----------------------------------------------------------------
> > altern |
> > eink | .000631 .0001332 4.74 0.000 .00037 .0008921
> > AB | (dropped)
> > -------------+----------------------------------------------------------------
> > ET | (base alternative)
> > -------------+----------------------------------------------------------------
> > AL |
> > AB | 6.498919 2.957141 2.20 0.028 .7030285 12.29481
> > _cons | -1.80445 .1309075 -13.78 0.000 -2.061024 -1.547876
> > -------------+----------------------------------------------------------------
> > R |
> > AB | -4.308268 2.407709 -1.79 0.074 -9.02729 .4107547
> > _cons | -1.236563 .0867427 -14.26 0.000 -1.406576 -1.066551
> > -------------+----------------------------------------------------------------
> > KS |
> > AB | 15.49664 1.622816 9.55 0.000 12.31598 18.6773
> > _cons | -.1677057 .0752585 -2.23 0.026 -.3152096 -.0202019
> > -------------+----------------------------------------------------------------
> > /lnsigmaP1 | .0504696 . . . . .
>
> The call to -asmprobit- is
>
> . asmprobit wahl eink AB, case(z1) alternatives(altern) casevars(AB)
> > correlation(pattern B)
>
> Barbara is including a case-specific variable, AB, in two places
> in her model specification: the -indepvars- variable list and
> the -casevars()- variable list. This really should not be allowed
> and -asmprobit- should have terminated with an error.
>
> Since AB is case-specific Barbara will have to drop it from the
> -indepvars- variable list.
>
> . asmprobit wahl eink, case(z1) alternatives(altern) casevars(AB)
> > correlation(pattern B)
>
>
> I hope this helps.
>
> -Rich
> [email protected]
>
>
> *
> * For searches and help try:
> * http://www.stata.com/support/faqs/res/findit.html
> * http://www.stata.com/support/statalist/faq
> * http://www.ats.ucla.edu/stat/stata/
> -- -- -- -- -- -- -- -- -- -- -- -- -- --
> Dipl.-Volksw. Barbara Hanel
> FAU Erlangen Nürnberg
> Lehrstuhl für Statistik und empirische Wirtschaftsforschung; Prof. Riphahn,
> PhD
> 0911/5302-258
> Lange Gasse 20
> 90403 Nürnberg
> [email protected]
--

-- -- -- -- -- -- -- -- -- -- -- -- -- --
Dipl.-Volksw. Barbara Hanel
FAU Erlangen N�rnberg
Lehrstuhl f�r Statistik und empirische Wirtschaftsforschung; Prof. Riphahn,
PhD
0911/5302-258
Lange Gasse 20
90403 N�rnberg
[email protected]
*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/








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