hi all,
In the xtlogit regression below 17667 groups of data have been dropped.
(All variables in this regression are dummy variables.)
1. Why have this data been dropped?
2. What features did these data have?
3. Is it possible that the selection bias has occurred despite the dropped of most of the data?


Code:
. xtlogit underline L.q L.underline,fe
note: multiple positive outcomes within groups encountered.
note: 17,667 groups (35,334 obs) dropped because of all positive or
      all negative outcomes.

Iteration 0:   log likelihood = -1074.9136  
Iteration 1:   log likelihood = -939.89854  
Iteration 2:   log likelihood = -937.31563  
Iteration 3:   log likelihood = -937.09315  
Iteration 4:   log likelihood = -937.09076  
Iteration 5:   log likelihood = -937.09028  
Iteration 6:   log likelihood = -937.09019  
Iteration 7:   log likelihood = -937.09019  

Conditional fixed-effects logistic regression   Number of obs     =      5,014
Group variable: Address                         Number of groups  =      2,507

                                                Obs per group:
                                                              min =          2
                                                              avg =        2.0
                                                              max =          2

                                                LR chi2(2)        =    1601.26
Log likelihood  = -937.09019                    Prob > chi2       =     0.0000

------------------------------------------------------------------------------
   underline |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
           q |
         L1. |   .0356927   .1197559     0.30   0.766    -.1990246      .27041
             |
   underline |
         L1. |  -20.42288   798.6675    -0.03   0.980    -1585.782    1544.937
------------------------------------------------------------------------------