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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 08:07:17 +0100 (BST)

That is hard to say from your original example, as you hid the
variables you used in a local macro. Anyhow if you estimate it that way
you will have to correct the standard errors as well. However, I don't
see why you would go through the trouble (and the possibility of adding
bugs) when you have a perfectly working program ready for you to use.

-- Maarten

--- Shehzad Ali <sia500@york.ac.uk> wrote:

> Thanks, Martin, but as I understand the two-part model that I am
> trying to
> estimate is little different from Heckman model because Heckman
> includes an
> Inverse Mills Ratio on the right hand side in the second part of the
> model
> whereas two-part model does not. That's what I understood from the
> following
> article:
> 
> Dow & Norton (2003). Choosing between and interpreting the Heckit and
> two-part models for corner solutions. Health Services and Outcomes
> research
> methodology. 4: 5-18. Page 10
> 
> Please feel free to correct me if I am wrong.
> 
> Regards,
> 
> Shehzad
> 
> 
> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu
> [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Maarten
> buis
> Sent: 16 August 2008 06:35
> To: statalist@hsphsun2.harvard.edu
> Subject: Re: st: stata code for two-part model
> 
> --- 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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-----------------------------------------
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/
-----------------------------------------

Send instant messages to your online friends http://uk.messenger.yahoo.com 
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