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st: CLAD error message and coefficient interpretation


From   Kevin McCourt <kev_mccourt@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: CLAD error message and coefficient interpretation
Date   Mon, 22 May 2006 11:36:14 -0700 (PDT)

Hi everyone,

I have two questions regarding STATA's CLAD procedure.

Firstly, I have a censored dependent variable--it is
fraction of plant capacity used. My data set is a
panel of 10 years tracking about 30 different timber
processing factories. Model specification requires
fixed effects and I definitely have heteroskedasticity
so tobit does not do what I need. I read Chay &
Powell's paper and I think they might just provide the
solution to my problem. When I input my data and run
the model, I get an error that states one of my dummy
variables that identifies one of the plants is
missing. The variable IS there. Depending on the model
specification (that is, what other explanatory
variables I include), I get the same error message but
for varying dummy variables. It is always the same
message--"factoryZ not found" (where factoryZ is the
name of the dummy). I have a suspicion this is caused
by collinearity, because when I remove some of the
variables the error is resolved. How can I know for
sure what is going wrong? This error happens
regardless of whether I use CLAD or CQREG (chay &
powell's version).

Also, how do I interpret the coefficients? Are they
likelihood ratios or are they in units of my dependent
variable? My dependent variable goes from 0 to 1.02. I
ask this because one of the coefficients is 1.2 and I
have a hard time understanding what it really implies.
Also, should I use the BIAS value and substract it
from the coefficient?

thank you all in advance,

Kevin McCourt

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