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st: RE: Re: use mfx to compute marginal effects in tobit


From   "Maarten Buis" <M.Buis@fsw.vu.nl>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Re: use mfx to compute marginal effects in tobit
Date   Wed, 8 Aug 2007 12:08:25 +0200

--- Zou Hong wrote:
> I estimate a tobit model and then use "mfx" to compute 
> marginal effects of variables. Strangely, the computed
> marginal effects for all the variables are the same as
> the estimated cofficient in the probit model.  It 
> seems "mfx" does not function.

There are a variety of marginal effects possible and of 
interest after -tobit-: 
o the effect on the expected value of the latent variable
o the effect on the expected value of the dependent, 
  variable conditional on it being larger than the lower 
  bound, and
o the effect on the probability of being larger than the 
  lower bound.

For more on this see: (Breen 1996).

By default Stata chooses the effect on the latent variable, 
which are exactly the same as the coefficients estimated by 
-tobit-. You will have to specify the -predict()- option in 
-mfx- to get the other marginal effects. See the example 
below, and -help mfx- and -help tobit postestimation-

*--------- begin example ----------
sysuse auto, clear
tobit mpg pric wei, ll(15)
mfx
mfx, pred(e(15,.))
mfx, pred(pr(15,.))
*----------- end example ----------
(For more on how to use examples I sent to the Statalist, see
http://home.fsw.vu.nl/m.buis/stata/exampleFAQ.html )

Hope this helps,
Maarten

Richard Breen (1996) "Regression Models: Censored, Sample 
Selected, or Truncated Data". Thousand Oaks: Sage.

-----------------------------------------
Maarten L. Buis
Department of Social Research Methodology 
Vrije Universiteit Amsterdam 
Boelelaan 1081 
1081 HV Amsterdam 
The Netherlands

visiting address:
Buitenveldertselaan 3 (Metropolitan), room Z434 

+31 20 5986715

http://home.fsw.vu.nl/m.buis/
-----------------------------------------

 



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