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st: Not clear if I need to use -heckman- once or twice

From   Hugh Colaco <[email protected]>
To   statalist <[email protected]>
Subject   st: Not clear if I need to use -heckman- once or twice
Date   Mon, 30 Nov 2009 09:07:20 -0500


This is the standard -heckman- example from the -help- menu using the
-twostep- procedure:-

webuse womenwk
heckman wage educ age, select(married children educ age) twostep

Suppose I want to take it one step further and predict the personal
wealth of women. This is not the best example, but it should be
somewhat clear as to what I am trying to achieve.

Let's assume "hours" (i.e. hours worked) is another variable in the
dataset and takes on a value only for women who earn a wage.

I then create an interaction term with "wage".

gen wagehrs = wage*hours

I need to run the following model:-

wealth = f(wage, hours, wagehrs, control variables)

I have data on the variable "wealth" and the control variables for
women who earn a wage AND women who don't.

Can someone please help me to model the above? Basically, I am not
clear if I need to run -heckman- once (as above) or twice? (ignore the
economic relevance or otherwise of the "wealth" regression since it is
merely for illustration purposes)


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