RE: st: Heckman Selection Model: How to deal with missing observations?

["jyotsna puri" <pjyotsna@hotmail.com>]

Quan,
I am not sure what you mean by the following:
> I think one of the reason is probably due to many missing observations.
Are you trying to say that the population of observations with the dependent 
variable
missing, is very large (how large?). Or is this for the independent 
variables.
Also in estimating a probit, and then using the estimated dependent variable 
into
the OLS, you have to a/c for the correlation of the error terms, which is 
what the
heckman procedure essentially helps you do.
I am afraid unless you give more details, its extremely hard to answer your 
questions.
-Jo




Jyotsna (Jo) Puri
pjyotsna@hotmail.com
Doctoral Candidate, Department of Agriculture and Resource Economics,
University of Maryland, College Park, MD
I ran the Marine Corp Marathon to help educate.




----Original Message Follows----
From: Quang Nguyen <quangn@gmail.com>
Reply-To: statalist@hsphsun2.harvard.edu
To: statalist@hsphsun2.harvard.edu
Subject: st: Heckman Selection Model: How to deal with missing observations?
Date: Thu, 17 Feb 2005 12:01:07 -1000

Dear All,

I am applying the Heckman procedure to estimate the role of "Share
Contract" in longline fishery. In the first step, I run a Probit model
where the dependent variable is the binary variable "Share Contract or
something else", the independent variables include  economic factors
such as variable cost, risk sharing attitude...  In the second step, I
run OLS where dependent variable is revenue and several independent
variables including the binary variable "Share Contract...". My data
includes about 95 observations.

After running the Heckman procedures, no variable seems statisitically
significant. Some coeficinces have unxpected sign. I think one of the
reason is probably due to many missing observations. Or it could be
some other reasons.

I would highly appreciate if anyone could help me to fin dthe way
dealing with this matter, especially with respect to missing
observations. Also, any idea on how to improve the model would be very
much appreciated.

Have A Wonderful Day!
Quang
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