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Re: st: goodness of fit measure fir ivtobit


From   Austin Nichols <austinnichols@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: goodness of fit measure fir ivtobit
Date   Mon, 24 Sep 2012 12:38:50 -0400

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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