data( Mroz87 )
wooldridge <- heckit( log( wage ) ~ educ + exper + I( exper^2 ),
lfp ~ nwifeinc + educ + exper + I( exper^2 ) + age + kids5 + kids618, Mroz87 )
summary( wooldridge ) # summary of the 1st step probit estimation
# (Example 17.1, p. 562f) and 2nd step OLS regression
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