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Re: st: ZOIB procedure


From   Prerna S <maruiprerna@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: ZOIB procedure
Date   Wed, 21 Sep 2011 06:19:27 -0400

Thanks for clarifying the procedure, Maarten. Your emails have been
very helpful!

Prerna

On 20 September 2011 06:38, Maarten Buis <maartenlbuis@gmail.com> wrote:
> On Tue, Sep 20, 2011 at 12:19 PM, Prerna S <maruiprerna@gmail.com> wrote:
>> What i really should have asked is how does the marginal effects
>> option in zoib combine the effects of all 2-3 equations in the model?
>> Heckman (I understand it's a different procedure and not really
>> comparable) uses a mills ratio to do so. Does zoib do something
>> similar?
>
> No, this is also why I warned you not to think of -zoib- as a
> selection model. In principle you can estimate a zoib by just
> estimating a beta-model for the part between 0 and 1, and a logit for
> the exact 0s and exact 1s. -zoib- assumes that these two processes are
> completely separate. However, by estimating them in one model you can
> compute a expected proportion that includes the inflation parts, and
> that is what the default prediction is, and through that becomes the
> default for marginal effects. However, for you this is irrelevant as
> you said earlier that you do not to care about this expected
> proportion but only about the conditional proportion and the
> probability of having an exact 0.
>
> Below is an example that shows that the equations in -zoib- are
> completely separate.
>
> *--------------------- begin example ----------------------
> // get data
> use http://fmwww.bc.edu/repec/bocode/k/k401.dta, clear
> replace totemp = totemp/100
>
> // estimate zoib as one model
> zoib prate mrate totemp age sole,   ///
>     oneinflate( mrate totemp age sole)
>
> // Indication 1:
> // the covariances of the proportion and oneinflate equation
> // and oneinlate and ln_phi equation are effectively 0
> matlist e(V)
>
> // Indication 2:
> // estimate zoib in two separate parts leads to exactly
> // the same estimates
> betafit prate , mu(mrate totemp age sole)
> gen one = prate == 1 if prate < .
> logit one mrate totemp age sole
> *---------------------- end example -----------------------
>
> -- Maarten
>
> --------------------------
> Maarten L. Buis
> Institut fuer Soziologie
> Universitaet Tuebingen
> Wilhelmstrasse 36
> 72074 Tuebingen
> Germany
>
>
> http://www.maartenbuis.nl
> --------------------------
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