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

["Zou Hong" <hzou@ln.edu.hk>]

Dear Maarten,

Many thanks for your kind suggestions and the problem is resovled now.

Zou Hong
----- Original Message ----- 
From: "Maarten Buis" <M.Buis@fsw.vu.nl>
To: <statalist@hsphsun2.harvard.edu>
Sent: Wednesday, August 08, 2007 6:08 PM
Subject: st: RE: Re: use mfx to compute marginal effects in tobit



> --- 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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