<>
You have to leave "mill" out of your -predict-ion...
**************
use http://www.stata-press.com/data/r11/womenwk.dta, clear
heckman wage educ age, /*
*/ select(married children educ age)/*
*/ twostep mills(mill)
predict yheck
reg wage educ age mill
gen yreg=_b[_cons]+_b[educ]*educ + /*
*/ _b[age]*age
su y*
compare yheck yreg
**************
HTH
Martin
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of [email protected]
Sent: Montag, 28. September 2009 19:38
To: statalist
Subject: st: How to understand the linear prediction after -heckman-?
Dear statalists,
In my mind, the result of -heckman- is the same as the result of -reg- with
lambda as an additional independent variable. However, I found something
different.
I typed the following in the stata, and the related results are as
followings.
. qui heckman wage educ age, select(married children educ age) twostep
mills(mill)
. qui predict double yheck if wage!=.
. qui corr yheck wage if wage!=.
. di r(rho)^2
.2512235
. qui reg wage educ age mill if wage!=.
. qui predict double yreg if wage!=.
. qui corr yreg wage if wage!=.
. di r(rho)^2
.27934983
. su yheck yreg
Variable | Obs Mean Std. Dev. Min Max
-------------+--------------------------------------------------------
yheck | 1343 21.96174 3.717985 14.79669 32.88486
yreg | 1343 23.69217 3.332614 16.22861 33.78897
The predicted value should be the same and the R-squared should be the same
too.
Why different?
Hope for any help and thank you very much.
Best regards,
Rose.
*
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*
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* http://www.ats.ucla.edu/stat/stata/
