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Re: st: ZOIB procedure

From   Maarten Buis <[email protected]>
To   [email protected]
Subject   Re: st: ZOIB procedure
Date   Tue, 20 Sep 2011 12:38:58 +0200

On Tue, Sep 20, 2011 at 12:19 PM, Prerna S <[email protected]> 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, 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
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