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st: Marginal Effects for ZINB


From   Chris Roebuck <Chris.Roebuck@Advancepcs.com>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   st: Marginal Effects for ZINB
Date   Wed, 12 Feb 2003 10:13:35 -0600

I am running ZINB (zero-inflated negative binomial) models, and after which
running MFX to calculate marginal effects.

Although several coefficient estimates are NEGATIVE, Stata reports POSITVE
marginal effects.

My question: Is this possible and why? Is this the case since the marginal
effect is for the overall model (which includes the zero-inflation equation)
and therefore if both coefficents from the two equations are negative, then
the resulting marginal effect if positive (i.e., it's a product of the two)?

One point on my models: The do not include instruments; both equations
contains the same variables, thus nonlinearities are identfying the
zero-inflation.

Thanks for your help.

*****************************
M. Christopher Roebuck, MBA
Health Economist
AdvancePCS
chris.roebuck@advancepcs.com
410-229-8382 office
410-785-8140 fax
*****************************

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