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RE: st: RE: xttobit initial value not feasible


From   "Lachenbruch, Peter" <Peter.Lachenbruch@oregonstate.edu>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: RE: xttobit initial value not feasible
Date   Tue, 15 Apr 2008 08:39:19 -0700

There are also two types of models that haven't been discussed:
Two-part or hurdle models - these estimate the probability of response
using a logistic model and estimate the magnitude of response among the
responders using a regression model.  This implies two things - the
responses are normal, and you can identify the zeros.  The last would be
something like finding hospital expenses in an insurance record, when
the insured has not been hospitalized.
The other model would be a mixture model of zeros and non-zeros.  If you
can't identify the zeros from theory, you can use the mixture model and
estimate the proportion of zeros.  The zip and zinb models are
representatives of this.  I don't recall seeing a zimr (zero-inflated
multiple regression) 
These are quite close in spirit, but may be far apart in estimation.

Tony

Peter A. Lachenbruch
Department of Public Health
Oregon State University
Corvallis, OR 97330
Phone: 541-737-3832
FAX: 541-737-4001


-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Austin
Nichols
Sent: Monday, April 14, 2008 11:46 AM
To: statalist@hsphsun2.harvard.edu
Subject: Re: st: RE: xttobit initial value not feasible

I doubt the large number of zeros is the ulitimate cause of your error
message.  There is probably something odd about your data.  Can you
run a pooled model with -tobit- instead of -xttobit-?  You may also
want to consider -poisson- and others in that family.

On Mon, Apr 14, 2008 at 2:24 PM, sarah young
<sarahyoungnew@googlemail.com> wrote:
> Nick,
>
>  Thanks so much for your reply again. you are right that 2/3 firms do
>  not export anything in my data, i.e. their export intensity is zero.
>  export intensity is measued as the ratio of exports and total sales.
>  Many existing literature on determinants of export intensity use
>  Tobit, I understand a reason is that they treat export intensity as a
>  censored variable. I read somewhere that for a variable like this,
>  i.e. with over half of the observation as zero, Tobit model is more
>  appropriate than xtreg or xtabond2. Is that right? Thanks a lot.
>
>  Sarah
>
>
>
>  On Mon, Apr 14, 2008 at 6:48 PM, Nick Cox <n.j.cox@durham.ac.uk>
wrote:
>  > My point had I think nothing to do with censoring, but with the
geometry
>  > of what you are
>  > asking Stata to do.
>  >
>  > Please appreciate that I don't know anything about your data, nor
>  > I am an economist.
>  >
>  > If I see that 2/3 of firms have zero "export intensity", then
unless
>  > intensity
>  > has some special meaning, I infer from that that 2/3 firms do not
export
>  > anything -- at least
>  > in the data period.
>  >
>  > sarah young
>  >
>  >
>  > Thanks for the response. I am just starting to learn econometrics.
I
>  > thought that for a variable with over half of the observations are
>  > zero, like export intensity in my model, the variable could be
treated
>  > as a censored variable. Or probably I am wrong?
>  >
>  > I have used xtreg and xtabond2 to estimate the model and they
performed
>  > well.
>  >
>  >
>  >
>  > On Mon, Apr 14, 2008 at 5:31 PM, Nick Cox <n.j.cox@durham.ac.uk>
wrote:
>  > > Consider a standard regression of your response (call it y) on
any two
>  > > predictors.
>  > > Then a simplified analogue of your problem is that two-thirds of
your
>  > > data lie on
>  > > the plane y = 0 and the other third lies on one side of that
plane.
>  > > Geometrically that sounds
>  > > a pretty awkward situation to model. Having more predictors and
using
>  > > tobit don't I think
>  > > make it any easier.
>  > >
>  > > There is no positive suggestion embedded here, just a thought
that the
>  > > message doesn't sound that
>  > > surprising in such a light.
>  > >
>  > > Nick
>  > > n.j.cox@durham.ac.uk
>  > >
>  > > sarah young
>  > >
>  > > I have been trying to run xttobit without success. The problem is
>  > > "initial value not feasible".
>  > >
>  > > I have a panel of more than 4000 firms for 15 years, which
altoghther
>  > > is about 62,000 firm-year observations. The dependent variable is
>  > > export intensity, two thirds of which are zeros. So I chose to
use
>  > > Tobit model. The model is on the determinants of export
intensity,
>  > > theoretically the choosing of the independent variables is fine,
which
>  > > include TFP, age, size, R&D intensity etc, as well as year and
>  > > industry dummies. I have tried by using different sets of
independent
>  > > variables, for example, including only one variable on the right
side,
>  > > excluding year and industry dummies, all of these not working and
the
>  > > message was the same, "initial value not feasible."
>  > >
>  > > I have searched on Statalist archive and found this thread:
>  > > http://www.stata.com/statalist/archive/2007-05/msg01036.html
>  > >
>  > > I tried to solve the problem according to the suggestions in the
>  > > thread but still it is not working. Could anyone give some
advice?
>  > > What are the possible problems behind this? How to solve them?
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