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Re: st: mfx


From   "Joao Ricardo F. Lima" <jricardofl@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: mfx
Date   Sat, 19 Jul 2008 14:01:26 -0300

Arne,

thanks for your answer. Now I understand that x represents the mpg
variable. I was curious because I had changed the code to:

#delimit ;
twoway (function y = _b[mpg]*normden(_b[price]*`mean_price'+
_b[mpg]*mpg+_b[_cons]), range(mpg)) ;
#delimit cr

and the graph generated was different.

Best regards,

Joao Lima

2008/7/19 Arne Risa Hole <arnehole@gmail.com>:
> Joao,
>
> What the code below does is plotting the marginal effect of the mpg
> variable evaluated at different values, holding the price variable
> constant at its mean. The x in the code represents the mpg variable -
> see -help twoway function-.
>
> Perhaps this is clearer:
>
> sysuse auto
> probit foreign price mpg
> mfx
> sum price
> local mean_price = r(mean)
> #delimit ;
> twoway (function y = _b[mpg]*normden(_b[price]*`mean_price'+
> _b[mpg]*x+_b[_cons]), range(mpg) ytitle(Marginal effect)
> xtitle(Miles per gallon)) ;
> #delimit cr
>
> Arne
>
> 2008/7/19 Joao Ricardo F. Lima <jricardofl@gmail.com>:
>> Dear,
>>
>> sorry the simple question. I am curious why I have to use "x" instead
>> mpg (mean) in the twoway graph (..._b[mpg]*x+_b...)? Or better, what
>> Stata understand when I type x in this command?
>>
>> Thanks a lot,
>>
>> Joao Ricardo
>>
>> 2008/7/18 Arne Risa Hole <arnehole@gmail.com>:
>>> Hi Chiara,
>>>
>>> How about this?
>>>
>>> sysuse auto
>>> probit foreign price mpg
>>> mfx
>>> sum price
>>> local mean_price = r(mean)
>>> #delimit ;
>>> twoway (function y = _b[mpg]*normden(_b[price]*`mean_price'+
>>> _b[mpg]*x+_b[_cons]), range(mpg)) ;
>>> #delimit cr
>>>
>>> This plots the marginal effect of the mpg variable over its range
>>> following a probit regression. The logic is the same with -biprobit-
>>> but the formula for the marginal effect will of course be a little
>>> more intricate in that case.
>>>
>>> Arne
>>>
>>> 2008/7/18 Chiara Mussida <cmussida@gmail.com>:
>>>> Dear All,nuous variab
>>>> is there a way to "plot" the mfx of a continuous variable (after a
>>>> biprobit) to infer its behaviour over the complete range of values and
>>>> not only for the mean of the independent variables (which is the
>>>> default)?
>>>>
>>>> Thanks
>>>>
>>>> --
>>>> Chiara Mussida
>>>> PhD candidate
>>>> Doctoral school of Economic Policy
>>>> Catholic University, piacenza (Italy)
>>>> *
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>>
>>
>>
>> --
>> -------------------------------
>> Joao Ricardo Lima
>> Professor
>> UFPB-CCA-DCFS
>> +553138923914
>> -------------------------------
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>



-- 
-------------------------------
Joao Ricardo Lima
Professor
UFPB-CCA-DCFS
+553138923914
-------------------------------
*
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*   http://www.ats.ucla.edu/stat/stata/



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