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From |
Lisa Powell <[email protected]> |

To |
[email protected] |

Subject |
Re: st: Re: Tobit coefficients |

Date |
Thu, 30 Dec 2004 12:55:47 -0500 (EST) |

Greetings, Following up on this earlier exchange over the marginal effects from the tobit model, I have a question regarding the STATA output: For the following command: tobit facil4p $demo5, ll(0); mfx compute, predict(e(0,.)); does this give marginal effects in terms of 1) E(y|x,y>0) OR 2) E(y|x) = P(y>0) * E(y|x,y>0) If it is (1) could someone please tell me how to specify (2). Many thanks, Lisa On Tue, 23 Mar 2004, Scott Merryman wrote: > Stefan, > > The output of -tobit- gives the estimated coefficients that are related to the > latent variable y*. It shows the effect of a change in a given x variable on the > expected value of the latent variable, holding all other x variables constant. > In Stata Manual example, the OLS coefficients of the latent model are compared > to the estimated latent effects of the tobit model E[y*|x]. But this is just > for illustration (see technical note, page 253) is you had uncensored sample > there would be no reason to use a tobit model. > > If you are comparing the OLS marginal effects estimated on the observed data, > than you would want use the expected unconditional of the observed data E[y|x]. > Usually, the marginal effects of the latent variable, y*, are not of interest > since it is unobserved -- we only observe y* if it above a threshold. Though > why would you want to compare them to OLS coefficients, since the OLS > coefficients will be inconsistent? > > Hope this helps, > Scott > > ----- Original Message ----- > From: "LACHENMAIER" <[email protected]> > To: <[email protected]> > Sent: Tuesday, March 23, 2004 8:11 AM > Subject: st: Tobit coefficients > > > > Hi all, > > > > I have some problems with the output of the Tobit command. I always > > thought that the coefficients presented after the tobit command are the > > "pure" tobit coefficients, i.e. if I want to compare them to OLS > > coefficients I have to rescale them by an adjustment factor (which > > depends on whether I want to get the unconditional effect E(y | y,x), or > > the conditional effect E(y | y>0,x), like described e.g. in Wooldridge > > (2003, ch. 17.2). > > > > But then I had a closer look at the Stata manual (Stata 8) because I > > wanted to learn more about the possibilities with mfx compute, > > predict(). But in the first tobit example in the manual (p.252) the > > tobit coefficient is directly compared to the OLS coefficient. So I am a > > bit confused what the output of the tobit command really is now and how > > to calculate which effects.... > > > > Maybe someone can help me here. > > Thanks in advance > > > > Stefan > > > > PS: Sorry for posting this problem again, but I have not solved the > > problem yet. Thanks! > > > > * > > * For searches and help try: > > * http://www.stata.com/support/faqs/res/findit.html > > * http://www.stata.com/support/statalist/faq > > * http://www.ats.ucla.edu/stat/stata/ > > * > * For searches and help try: > * http://www.stata.com/support/faqs/res/findit.html > * http://www.stata.com/support/statalist/faq > * http://www.ats.ucla.edu/stat/stata/ > Lisa M. Powell Research Associate Professor, University of Illinois at Chicago Fellow, School of Policy Studies, Queen's University tel: 1-312-413-8468; fax: 312-355-2801 email: [email protected] [email protected] * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**st: update problem***From:*"Barbara Hamilton" <[email protected]>

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