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From |
Austin Nichols <austinnichols@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Dependent variable in a tobit model |

Date |
Mon, 15 Nov 2010 11:31:23 -0500 |

Estrella Gomez <estrellastata@gmail.com> : Why not use -poisson- or -glm- with cluster-robust standard errors (and fixed effects if you like)? Cameron and Trivedi would have you "trick" Stata's -tobit- command using a negative y in place of ln(zero), but I argue in http://repec.org/bost10/nichols_boston2010.pdf that tricking Stata this way is a bad idea; see also Santos Silva and Tenreyro. 2006. "The Log of Gravity." Review of Economics and Statistics, 88(4): 641-658. Cameron and Trivedi. 2009. Microeconometrics Using Stata. Stata Press, College Station TX. Note that a lot of zeros is not a problem in general, since those observations may also have very low predicted values conditional on observables. On Mon, Nov 15, 2010 at 5:19 AM, Estrella Gomez <estrellastata@gmail.com> wrote: > Dear Statalist: > > I am estimating a gravity equation of exports and I have decided to > use tobit model, since the number of zeros in the dataset is high. > This model is usually estimated in its loglinear version. However, if > I use the logarithm of trade as dependent variable, the zeros are > dropped (since the logarithm of zero is unfeasible). > > This is the command I have introduced in Stata 11: > > xttobit lxi lGDP_i lGDP_j contig comla smctry ldist RTAboth i.exporter > i.importer i.year, ll(0) > > where "lxi" is the logarithm of exports and the independent variables > are the standard variables in a gravity equation > > Of course, the number of censored observations is zero, since the > dependent variable is introduced in logs. I think this is not correct, > but I have seen this specification in most of the gravity-related > articles > > Could somebody help me with this? > > In addition, I do not know if it is correct to introduce exporter, > importer and time effects in the model > > Thank you in advance, > Estrella Gómez * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: st: Dependent variable in a tobit model***From:*Estrella Gomez <estrellastata@gmail.com>

**References**:**st: Dependent variable in a tobit model***From:*Estrella Gomez <estrellastata@gmail.com>

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