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st: some dummy variables are dropped by stata


From   "G.B.Li" <tslee1912@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: some dummy variables are dropped by stata
Date   Sun, 18 May 2008 22:32:28 -0700 (PDT)

Dear all:

I use dummy variables to control for individual fixed effects. some of the dummy variables are dropped by stata when i run OLS. for example:

......

_Iname_11 |   .0325197   .1472331     0.22   0.826                 .0485629
_Iname_12 |  -.2413537   .0841713    -2.87   0.005                -.1367784
_Iname_13 |  (dropped)
_Iname_14 |  (dropped)
_Iname_15 |  (dropped)
_Iname_16 |  (dropped)
_Iname_17 |  -.1292328   .1158529    -1.12   0.268                -.1780356

......

it is an unbalanced panel dataset, so i guess this is because these individuals do not have enough observations over time, right?

i am wondering:

1. how does stata deal with these individual and observations? drop them?
2. how does this phenomenon affect the final results (unbiased and consistent, etc.)? 

many thanks!

GB

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