RE: st: Dependent variable in a tobit model

[Nick Cox <n.j.cox@durham.ac.uk>]

Incidental detail: Austin's presentation never went anywhere near Microsoft PowerPoint. It was developed using LaTeX. 

As I understand it, -tobit- ignores missings. Regardless of that, I am not clear why you would expect -tobit- to understand that (some or all of) your missings really mean zero, or how you expect -tobit- to interpret missings in any sense. 

Nick 
n.j.cox@durham.ac.uk 

Estrella Gomez

The thing is that I am comparing estimation methods to stduy the
properties of each of these methods, that's why I am interested in
estimating a Tobit model, as well as Poisson and others. But I am not
sure if Stata automatically drops out the observations of the
dependent variable that are missings (those are the observations that
are zeros in the original variable in levels) and hence, I should
specify instead an equation with the dependent variable in levels
instead of logs. Is that correct?

Thank you very much for your advices and your ppt. Could I ask you
which is the command to plot the graphic in which you compare the
estimators?

Austin Nichols 


> Why not use -poisson- or -glm- with cluster-robust standard errors
> (and fixed effects if you like)?  Cameron and Trivedi would have you
> "trick" Stata's -tobit- command using a negative y in place of
> ln(zero), but I argue in
> http://repec.org/bost10/nichols_boston2010.pdf
> that tricking Stata this way is a bad idea; see also
> Santos Silva and Tenreyro. 2006. "The Log of Gravity." Review of
> Economics and Statistics, 88(4): 641-658.
> Cameron and Trivedi. 2009. Microeconometrics Using Stata. Stata Press,
> College Station TX.
>
> Note that a lot of zeros is not a problem in general, since those
> observations may also have very low predicted values conditional on
> observables.
>
> On Mon, Nov 15, 2010 at 5:19 AM, Estrella Gomez <estrellastata@gmail.com> wrote:

>> I am estimating a gravity equation of exports and I have decided to
>> use tobit model, since the number of zeros in the dataset is high.
>> This model is usually estimated in its loglinear version. However, if
>> I use the logarithm of trade as dependent variable, the zeros are
>> dropped (since the logarithm of zero is unfeasible).
>>
>> This is the command I have introduced in Stata 11:
>>
>> xttobit lxi lGDP_i lGDP_j contig comla smctry ldist RTAboth i.exporter
>> i.importer i.year, ll(0)
>>
>> where "lxi" is the logarithm of exports and the independent variables
>> are the standard variables in a gravity equation
>>
>> Of course, the number of censored observations is zero, since the
>> dependent variable is introduced in logs. I think this is not correct,
>> but I have seen this specification in most of the gravity-related
>> articles
>>
>> Could somebody help me with this?
>>
>> In addition, I do not know if it is correct to introduce exporter,
>> importer and time effects in the model

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