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Re: st: stata code for two-part model


From   Maarten buis <maartenbuis@yahoo.co.uk>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: stata code for two-part model
Date   Sat, 16 Aug 2008 06:34:43 +0100 (BST)

--- Shehzad Ali <sia500@york.ac.uk> wrote:
> I was wondering if someone can help with stata code for calculating
> marginal effects after two-part models for say, cost of care. Here,
> first part is a probit model for seeking care or not, and the second
> part is an OLS model of cost of care, conditional on decision to seek
> care. 

Looks to me you are trying to reinvent -heckman-. If you use -heckman-
you will both do a better job at modeling the process, and you will
have many -predict- option which you can feed into -mfx-, so you can
look at the various types of marginal effects that are possible in this
type op models. 

> Here is the simplified code:
> 
> probit care $xvar
> 
> reg cost $zvar if care==1
> 
> mfx
> 
> I understand that mfx after the second part gives us the marginal
> effects for the OLS part only, and not the conditional marginal
> effects.

The OLS part is already conditional, the problem here is that this is
not a two-part model as you are not using anything from the -probit-
model, in other words you could just as well leave that -probit- away
and your results will not change. I am not sugesting that you do that
though, you have selection on the y and you will have to model it. It
is just that the model you are proposing just isn't the way to do it.
Use -heckman- instead. 

Hope this helps,
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/
-----------------------------------------

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