Statalist


[Date Prev][Date Next][Thread Prev][Thread Next][Date index][Thread index]

Re: st: estimating the covariance matrix of probit two stage procedure


From   Maarten buis <[email protected]>
To   [email protected]
Subject   Re: st: estimating the covariance matrix of probit two stage procedure
Date   Mon, 24 Dec 2007 09:24:35 +0000 (GMT)

--- "Armelini, Guillermo" <[email protected]> wrote:
> I'm trying to estimate a recursive simultaneous equation using a
> probit model in the selection stage and a conditional censored
> regression in the second stage. Specifically, the model is the
> following one:
>  
> Z(i)=AV(i)+U(i) (equation 1)
> 
>  Z(i)=1 if Z(i)>0
> 
>  Z(i)=0 if Z(i)<=0
>  
> where A is a vector of parametes and V is a set of covariates for
> each (i)
>  
> Then I calculate Lambda which is the inverse mills of the predicted
> values of equation one for those observations in which the Dependent
> Variable is equal to 1
>  
> The second step is to calculate the tobin model:
>  
> Y(i)=bX(i)+E(i) if Z(i)=1  (second equation)
>  
> In this equation "bs" are the coefficients to be estimated and "X" is
> a vector of covariate which includes Lambda (which is the inverse
> mills of the predicted values of equation one in which the DV is = 1)
>  
> My question is how can I correct the covariance and the variance of
> the parameters since, Lee, Maddala  and Trost (1980) suggest that
> everytime a two stage model like the one I describe before is
> estimated, a procedure to correct the variance and covariance of the
> parameters should be run. 
>  
> Is anybody aware about how to do this in Stata?

Yes:
heckman Y X, select(V) twostep
X should not include lambda (Stata does that for you)

Best whishes (to you and everybody else on the list),
Maarten

-----------------------------------------
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/
-----------------------------------------


      ___________________________________________________________
Support the World Aids Awareness campaign this month with Yahoo! For Good http://uk.promotions.yahoo.com/forgood/
*
*   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/



© Copyright 1996–2024 StataCorp LLC   |   Terms of use   |   Privacy   |   Contact us   |   What's new   |   Site index