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RE: st: RE: xttobit initial value not feasible


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: RE: xttobit initial value not feasible
Date   Mon, 14 Apr 2008 19:38:05 +0100

That question is for others nearer your field to comment on. 

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of sarah young
Sent: 14 April 2008 19:25
To: statalist@hsphsun2.harvard.edu
Subject: Re: st: RE: xttobit initial value not feasible

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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