Dear list-members,
If I have the following Heckman selection model:
y = a + b.X + e (2nd stage)
z = c + d.W + u (selection equation)
Would you know whether X and W have to be completely
distinct?
I thought they ought to be distinct but I have seen some
papers that use variables that are common to both X and W.
Does anyone have a good answer or know of a definitive
source on this?
Thanks for your suggestions.
Nishant
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