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st: AW: Sign of Lambda in heckman

From   "Martin Weiss" <>
To   <>
Subject   st: AW: Sign of Lambda in heckman
Date   Mon, 1 Mar 2010 14:53:27 +0100


As [R], p. 651 says: lambda is the product of rho and sigma. If all you are
interested in is the sign, then sigma is irrelevant, since it will always be
positive, being a standard deviation. So you are really asking for
information about rho: I would check the references at the beginning of the
"Methods and formulas" for more insights about its meaning.


-----Ursprüngliche Nachricht-----
[] Im Auftrag von Sascha Steffen
Gesendet: Montag, 1. März 2010 14:27
Betreff: st: Sign of Lambda in heckman

Dear All,

I was wondering about an economic interpretation of the sign of the lambda
in a heckman selection model. For example, I am interested in factors
reducing default rates of firms. I only observe the performance of the loan
once the loan is approved. I use a selection model to account for this. My
lambda from the heckman model turns out to be significant and negative. What
is the interpretation? In what direction is the bias? Can I interpret the
negative sign, that information has been used to screen and filter out bad
applicants? What do you think?

Best wishes,

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