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From |
nicola.baldini2@unibo.it |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: tobit probit censored data confusion |

Date |
Wed, 03 Oct 2007 18:07:03 +0200 |

You may have a look at Stata's FAQ at http://www.stata.com/support/statalist/faq. That on selection models, if I remember correctly. Basically: - Sample selection bias refers to problems where the y is observed only for a restricted, nonrandom sample. If you believe that the coefficients of the covariates differ according to the selection variable, then you need -heckman-. The selection equation requires to specify independent variables that affect the probability to observe y, but not its level estimated in the regression equation (conditional on a non-zero observation). If you assume that the independent variables affect both the incidence and the level of y, the Heckman model reduces to a tobit specification (-tobit-). - Under treatment-effects models (-treatreg-), the coefficients of the covariates are restricted to be the same for treated and non-treated. Note that only tobit has a built-in command for panel data in Stata 9 (-xttobit-). However, you may use -xtivreg- (or -xtivreg2- from ssc) in lieu of -heckman-. Also, you may have a look at http://ideas.repec.org/p/boc/usug07/12.html Nicola At 02.33 03/10/2007 -0400, you wrote: >Dear all, > >I have a question regarding statistical models. I have a panel data >with obs on people's background information (age, wealth from other >sources, etc), and wage if the person decides to work. A person may >choose to work, if so, there is an observable wage; if not, wage is 0. > >I am confused about what models to use. It looks like a standard tobit >model, with wage censored at 0. Below is an alternative way to model. > >A: Probit model of the decision to work or not. >B: For wage>0 observations, use OLS regression. > >I have seen both in practice. But while using two models, the authors >mention that they are estimating both model A and model B at the same >time. What does it mean "at the same time"? > >Bottom line: >If both tobit and the joint estimation of models A and B are >appropriate, which is better? > >Thanks for clarifying this for me! * * 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/

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