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st: Re: Stata Heckman and Mills ratio

From   Christopher Baum <>
To   seb nieto <>
Subject   st: Re: Stata Heckman and Mills ratio
Date   Thu, 24 Sep 2009 13:10:57 -0400


you can always create an r^2 measure from the squared correlation of actual and predicted observations used in the estimation. So something like

reg y...

heckman y...
predict double heckp if e(sample), xb
corr heckp y
di r(rho)^2

which should be comparable to the r^2 displayed from the regress.

Kit Baum   |   Boston College Economics and DIW Berlin   |
An Introduction to Stata Programming   |
An Introduction to Modern Econometrics Using Stata   |

On Sep 24, 2009, at 12:50 PM, seb nieto wrote:

Dear Kit,

I am running a regression in which one of the explanatory variables could have a "selection bias".
The basic equation is
reg y x1 x2 x3 x4     (equation 1)

However x2 has a selection bias problem that can affect the result of equation 1

I then run the following regression:
heckman y x1 x3 x4, select(x2 = Z1) twostep mills(lambda) (equation 2) in which Z1 are a set of explanatory variables (some of them different than x1, x3 and x4).

I would like however compare the R2 of the equation (1) with respect to equation (2). Can I then run the following regression and report the results of equation (3) instead of those of equation (2)?:

reg y x1  x3 x4  lambda   (equation 3)

Of course, lambda variable of eqaution 3 is estimated from equation 2.

Thank you very much for your help.

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