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


From   "Zou Hong" <hzou@ln.edu.hk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: Re: RE: Re: use mfx to compute marginal effects in tobit
Date   Thu, 9 Aug 2007 17:57:19 +0800

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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