Re: st: mfx failing with tobit?

[Roger Cremades Rodeja <ecosafor@gmail.com>]

Dear Maarten and Nick,

thanks for your valuable comments, my doubt is clear now,

gratefully,
Roger

2014-03-26 9:48 GMT+01:00 Maarten Buis <maartenlbuis@gmail.com>:
> To add to Nick's repetition of my answer:
>
> "The" marginal effect for a Tobit model does not exist: there are
> several marginal effects depending on what your outcome of interest
> is. The outcome of interest could be the expectation of the latent
> variable, the expectation of the censored variable where the censored
> values are replaced by 0 (i.e. the values one would actually observe
> in the field), the expectation of the censored variable conditional on
> not being censored, or the probability of being censored. To learn
> more about these you could look at (Breen 1996) The default is the
> expectation of the latent variable, and this is a linear function of
> the explanatory variables, so the these marginal effects will be the
> same as the coefficients. If you want one of the other marginal
> effects, you will have to specify the -predict()- option in -margins-.
> This will also require that you study the -help tobit_postestimation-
> helpfile, and probably the manual entry for -tobit- and -tobit
> postestimation-.
>
> Hope this helps,
> Maarten
>
> Richard Breen (1996) "Regression Models: Censored, Sample Selected, or
> Truncated Data." Thousand Oaks: Sage.
>
>
> On Tue, Mar 25, 2014 at 5:42 PM, Nick Cox <njcoxstata@gmail.com> wrote:
>> Maarten has already addressed this directly. As tobit is a linear
>> predictor, the derivatives are equal to the coefficients of the
>> predictors. It's like asking: if I differentiate the equation of a
>> hyperplane, will I get more information out? No; the information is
>> already given in the coefficients.
>>
>> Nick
>> njcoxstata@gmail.com
>>
>> On 25 March 2014 16:34, Roger Cremades Rodeja <ecosafor@gmail.com> wrote:
>>
>>> using stata 12, and trying to obtain the marginal estimates after a
>>> Tobit regression, I write:
>>>
>>> tobit indepvar depvar1 depvar2 .. depvar9, ll
>>> margins, dydx (*)
>>>
>>> the second command is supposed to provide the "estimate marginal
>>> effect of variables in varlist", after checking the help of margins,
>>> but I only get again the same coefficients as in the Tobit
>>> regressions, but labelled as dy/dx
>>>
>>> I do not know what should I do to obtain the marginal estimates of a
>>> Tobit, I understood that was the right way,
>>
>> 2014-03-17 17:30 GMT+01:00 Roger Cremades Rodeja <ecosafor@gmail.com>:
>>
>>>> it was extremely helpful, I will see what can I do with stata 10, or
>>>> use the 12th of a colleague,
>>
>> 2014-03-17 14:26 GMT+01:00 Maarten Buis <maartenlbuis@gmail.com>:
>>
>>>>> -mfx- is a general purpose program, so it needs to choose a default
>>>>> outcome for which to compute the derivative. It chooses that default
>>>>> to be the default chosen for the predict option of the command, which
>>>>> in the case of -tobit- happens to be the linear predictor. So the
>>>>> default marginal effect computed for -tobit- is just the coefficients.
>>>>> If you want to change that, you need to specify the -predict()- option
>>>>> to whatever outcome you are interested in.
>>>>>
>>>>> Notice however, that since you did not specify the version of Stata
>>>>> you are using we must, per the Statalist FAQ, assume you use the
>>>>> latest version of Stata (13). In that case the -mfx- command is
>>>>> hopelessly outdated, and you should use the -margins- command instead.
>>
>> On Mon, Mar 17, 2014 at 11:13 AM, Roger Cremades Rodeja
>>
>>>>>> after running a Tobit regression, I introduce the mfx command and
>>>>>> surprisingly I get exactly the same coefficient estimates I had from the
>>>>>> regression, although labelled as dy/dx. Does anybody had a similar problem?
>> *
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>
>
>
> --
> ---------------------------------
> Maarten L. Buis
> WZB
> Reichpietschufer 50
> 10785 Berlin
> Germany
>
> http://www.maartenbuis.nl
> ---------------------------------
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