Maarten,
Thank you for your suggestion. I will see if I have
any luck finding instrumental variables!
Regards,
Ana Rios
--- Maarten buis <[email protected]> wrote:
> This looks to me that you are not using instrumental
>
> variables to identify your model, i.e. you need to
> find a variable that influences market participation
> status but not sales/puchases. Than you need to
> exclude those variables from the indepvars list and
> include it in the indepvars_sel list (in the
> -select()- option).
>
> This need for instrumental variables is in my humble
>
> opinion a major weakness of these heckman-type
> models
> because such variables almost never exist...
>
> -- Maarten
>
> --- "Ana R. Rios" <[email protected]> wrote:
> > Thank you for your suggestions. The dependent
> > variable in my model is the volume of
> sales/purchases
> > and the selection variable is the market
> participation
> > status: 0=net buyer, 1=autarkic, 2=net seller. As
> > dependent variables in the model I am using
> household
> > and farm characteristics.
> >
> > I have been using the twostep estimation procedure
> and
> > experimented with different explanatory variables.
>
> >
> > I got the oheckman command in the official help
> files,
> > FAQs, Examples, SJs, and STBs. A detailed
> description
> > of the command is available in The Stata Journal
> 2007,
> > 7(2):1-16.
> >
> > Once again, any advise/suggestions are greatly
> > appreciated.
> >
> > Thank you for your time.
> >
> > Regards,
> >
> > Ana Rios
> > --- Maarten buis <[email protected]> wrote:
> >
> > > --- "Ana R. Rios" <[email protected]> wrote:
> > > > I am trying to use oheckman but I am getting
> the
> > > > following message:
> > > > "Warning: Estimate of rho1 = . is outside
> feasible
> > > > interval [-1, 1]."
> > > > I would appreciate any suggestions/insights on
> how
> > > to
> > > > overcome this problem.
> > >
> > > It seems that -oheckman- has "converged" to an
> > > infeasible solution. The solution is to estimate
> a
> > > better model, as you haven't told us anything
> about
> > > your model I can't be very specific. One thing
> that
> > > jumps to mind with any heckman-like model is to
> make
> > > sure your model is properly identified, i.e. use
>
> > > instruments instead of relying on non-linearity.
> > > Another thing you might want to look at is to
> use
> > > the -twostep- option and see if that leads to
> any
> > > better estimates.
> > >
> > > -- Maarten
> > >
> > > BTW. -oheckman- is not in official Stata, so
> tell
> > > us where you got it.
> > >
> > > -----------------------------------------
> > > 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/
> -----------------------------------------
>
>
>
>
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