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st: question about bivariate probit

From   "Sun, Yan (IFPRI)" <Y.SUN@CGIAR.ORG>
To   <>
Subject   st: question about bivariate probit
Date   Tue, 22 Jul 2008 17:02:43 -0400

Dear STATA list, 

I am now running bivariate probit model. The command is listed as
following (y1 is suspected endogenous):
"biprobit (y1 x1 x2 x3) (y2 y1 x1 x2 x3)" while both y1 and y2 are 0/1

I have three questions:
1) When I run univariate preliminary model "probit y2 y1 x1 x2 x3", the
coefficient of y1 is strongly significant, which meaning y1 does affect
y2; but when I run biprobit model, the coefficient of is y1 is
insignificant and rho is small (0.15) and insignificant. What is the
possible reason?
2) After I run biprobit, I type command "predict xb1, xb1", but I find
95% of the predicted value for y1 (xb1)  are negative, which is strange
because the probability (xb1 is supposed to be probability for y1)
should between 0 and 1, do I misunderstood something here?
3) I try to run sensitivity analysis for different rho ranging from 0.1
to 0.5 to estimate regression, does anybody knows how to do this? 

Thank you for your help. 


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