Re: st: goodness of fit measure fir ivtobit

["Anat (Manes) Tchetchik" <anatmanes@gmail.com>]

Austin hi,
I have read the material regarding the ivpois model you have referred
me to and ran it on my data set. The results look pretty plausible .
Just before sticking to this model I have few last questions:

1. Can I use the exposure variable (in my case the maximum of the two
values: respondent's adulthood years and and the no. of years his/her
relative is staying abroad) also as an independent var.? (or given
that I do it- I should not use the exposure option)

2. I'm not sure i understand when I should use the exposure option and
when the offset one

3. Do I calculate goodness of fit measure using the same procedure you
recommended earlier
(http://www.stata.com/support/faqs/statistics/r-squared/) ?

Thank you very much!
Anat

On Mon, Sep 24, 2012 at 6:38 PM, Austin Nichols <austinnichols@gmail.com> wrote:
>
> Anat (Manes) Tchetchik <anatmanes@gmail.com> :
> Indeed, no censoring in your model. You have exactly the case I
> referred to when I wrote "I suspect you have a lower limit at zero
> which is actually a very low conditional mean rounded down to zero."
> Unless you believe that the Tobit model somehow correctly captures
> bunching at zero due to utility functions which would imply a negative
> demand for travel abroad, if such a thing were possible, which is
> implausible at best, you are much better off assuming that people with
> zero travel abroad simply have very low demand, and a group that has a
> conditional mean of 1/100000 trips will indeed have a lot of zeros
> observed.  The -ivpois- package on SSC, and the -gmm- specifications
> that supersede it, are designed to allow instrumental variables in a
> count model, or a regression with nonnegative outcomes more generally:
> http://www.stata.com/meeting/boston10/boston10_nichols.pdf
> http://fmwww.bc.edu/repec/bocode/i/ivpois.html
>
> On Mon, Sep 24, 2012 at 10:54 AM, Anat (Manes) Tchetchik
> <anatmanes@gmail.com> wrote:
> > I haven't thought about the count model, I will definitely try to run
> > it! thanks much!
> >
> > On Mon, Sep 24, 2012 at 5:38 PM, Maarten Buis <maartenlbuis@gmail.com> wrote:
> >> That does not sound like censoring at all. I would think of this as a
> >> regular count model. There are examples on how to deal with such an
> >> iv-model in -help gmm-.
> >>
> >> Hope this helps,
> >> Maarten
> >>
> >> On Mon, Sep 24, 2012 at 4:11 PM, Anat (Manes) Tchetchik
> >> <anatmanes@gmail.com> wrote:
> >>> Austin Hi,
> >>> Thank you very much for your reply!
> >>> What I have as a dependent var. are 500 respondents' reports of the
> >>> number of times they travelled abroad to visit their friends and
> >>> relatives over the course of their adult lives.  Some respondents yet,
> >>> who have relatives abroad, did not travel at all.
> >>> So the observations are censored at zero, with mean =2.2, max =50 and
> >>> stdev= 3.8.
> >>> Do you think in that case that the general methods of moments will be better?
> >>> Thanks much!!!
> >>> Anat
> >>>
> >>> On Sun, Sep 23, 2012 at 5:49 AM, Austin Nichols <austinnichols@gmail.com> wrote:
> >>>>
> >>>> Anat (Manes) Tchetchik <anatmanes@gmail.com>:
> >>>> You can always -predict- and compute the squared correlation of
> >>>> predictions with observed values:
> >>>> http://www.stata.com/support/faqs/statistics/r-squared/
> >>>> but are you sure your -ivtobit- model is justified?  What is the
> >>>> process that results in observations being censored?  I suspect you
> >>>> have a lower limit at zero which is actually a very low conditional
> >>>> mean rounded down to zero--am I right?  You may be better off with a
> >>>> -gmm- model.
> >>>>
> >>>> On Sat, Sep 22, 2012 at 5:33 PM, Anat (Manes) Tchetchik
> >>>> <anatmanes@gmail.com> wrote:
> >>>> > Dear statalisters,
> >>>> >
> >>>> > I wonder if anyone knows any goodness of fit that is appropriate for
> >>>> > tobit with endogenous
> >>>> > variables (ivtobit). Not as in "regular" tobit, stata does not report any
> >>>> > goodness of fit measure, any idea how to estimate such a measure?
> >>>> > Any response will be greatly appreciated..
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--
Anat Tchetchik, PhD
Department of Hotel and Tourism Management
Guilford Glazer Faculty of Business and Management
Ben-Gurion University of the Negev
P.O.Box: 653
Beer-Sheva, Israel, 84105

E-mail:       anat@som.bgu.ac.il
Phone         972-(0)8-6479735
Fax:           972-(0)8-6472920
Web:          http://cmsprod.bgu.ac.il/Eng/som/hotelmanage/Staff/Academic/ChechikA.htm
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