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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 computedThere are a variety of marginal effects possible and of interest after -tobit-: o the effect on the expected value of the latent variable

marginal effects for all the variables are the same as

the estimated cofficient in the probit model. It seems "mfx" does not function.

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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**References**:**st: RE: Re: use mfx to compute marginal effects in tobit***From:*"Maarten Buis" <M.Buis@fsw.vu.nl>

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