Greetings,
I am estimating a model of distance traveled with the car on weekends,
which seems to bring up two possible sources of selectivity bias: 1.
does the household own a car and 2. do they use the car at all on the
weekend? Having controlled for sample selection in both cases, I'd like
to estimate distance traveled.
The following FAQ suggests a simple approach to this problem.
http://www.stata.com/support/faqs/stat/bias.html
The author writes:
"Finally, if Model II were desired, then one would be confronted with a
double selection model. I believe one would estimate a probit for labor
force participation first. Upon generating the IMR term, this would be
included in a second probit equation explaining union status. The
appropriate IMR term from this equation would then be included in the
two final wage equations. (This topic is covered in Amemiya 1985.)"
I plan to look into Amemiya, but in the meantime can anyone one confirm
whether this approach is sound?
Alternatively, rather than estimating two probits, it seems one could
also start by using heckprob, but I'm not sure how much sense it makes
to derive the Mills ratio from this model.
Any advice would be appreciated.
Many thanks,
Colin
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