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st: Is it necessary to have some variable for exposure in xtnbreg


From   Indu Khurana <indu.statauser@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: Is it necessary to have some variable for exposure in xtnbreg
Date   Tue, 14 Sep 2010 20:33:33 -0400

Dear Statalisters,

I have a count data on number of patents which is the dependent
variable (panel setting). Right hand side variables include Employee
well being and debt-equity ratio. The data on patents has a lot of
zeros. Although ZINB could have been an option but I am using fixed
effects. Therefore I decided to settle with negative binomial, fixed
effects - xtnbreg

My question is: Is it necessary to have some variable for exposure in xtnbreg?

The data is on 5000 firms for 5 years which all came into existence at
the same time in 2004. So basically they all have the same exposure.
And there is no information on month of the the year.
There is no other time dimension which can be used to fit in exposure.

I was thinking of the following options:
 1. To ignore exposure variable all together
     Which I am not sure of whether it is correct.
2. To include number of years of owner's education to fill as exposure.



Please suggest.

Thanks

Indu Khurana

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