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 <[email protected]> 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: [email protected]
> [mailto:[email protected]] On Behalf Of Maarten
> buis
> Sent: 16 August 2008 06:35
> To: [email protected]
> Subject: Re: st: stata code for two-part model
>
> --- Shehzad Ali <[email protected]> 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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* http://www.ats.ucla.edu/stat/stata/
