st: xtreg fixed effects

[Alexander James <alexandre_lille-paris@hotmail.com>]

Dear Statalist members,



I am running a a fixed effect models (xtreg) predicting the share of self citations that firms making in their patents. However, there is one thing that is worring me. I have some observations that appear just once in my database, but when I run the fixed effects they are not droped (they are considered in the model) so I get something like:

Fixed-effects (within) regression Number of obs = 315


Group variable: firm_id Number of groups = 159

R-sq: within = 0.3211 Obs per group: min = 1
between = 0.0512 avg = 2.0
overall = 0.0092 max = 13

F(27,158) = 16.53
corr(u_i, Xb) = -0.7005 Prob> F = 0.0000


Is this a mistake? how fixed effects model can run on observations that appear only once in the data?



Best Regards,


Alexandre 		 	   		  
*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/