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st: fixed effects for unbalanced panel


From   Deepak Hegde <hegde@haas.berkeley.edu>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: fixed effects for unbalanced panel
Date   Mon, 6 Oct 2008 22:29:17 -0700

I have an unbalanced panel (for most firms, I have two or more years of data, and for some firms only one year).

I want to know what stata does to observations (firms) with only one year of data when I run a fixed effects model with either the xtreg, fe(firm_id) or areg, absorb (firm_id) command.  It seems like the correct thing to do is to drop observations with only one year of data, but I am getting different results when I explicitly exclude firms with only one-year of data and when I use Stata's fixed effects commands.  This suggests that Stata is using data on firms without time-variation in a way that I don't understand...

Thank you much.

--- Deepak

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