. use if id != 407 & !missing(fte) using http://fmwww.bc.edu/repec/bocode/c/CardKrueger1994.dta, clear
(Dataset from Card&Krueger (1994))

. bys id: drop if _N!=2
(19 observations deleted)

. drop if id == 1 & t==0
(1 observation deleted)

. xtset id t
       panel variable:  id (unbalanced)
        time variable:  t, 0 to 1
                delta:  1 unit

. xtdes

      id:  1, 2, ..., 522                                    n =        389
       t:  0, 1, ..., 1                                      T =          2
           Delta(t) = 1 unit
           Span(t)  = 2 periods
           (id*t uniquely identifies each observation)

Distribution of T_i:   min      5%     25%       50%       75%     95%     max
                         1       2       2         2         2       2       2

     Freq.  Percent    Cum. |  Pattern
 ---------------------------+---------
      388     99.74   99.74 |  11
        1      0.26  100.00 |  .1
 ---------------------------+---------
      389    100.00         |  XX

. xtreg fte i.(treated bk kfc roys wendys t), re cluster(id)
note: 1.wendys omitted because of collinearity

Random-effects GLS regression                   Number of obs     =        777
Group variable: id                              Number of groups  =        389

R-sq:                                           Obs per group:
     within  = 0.0003                                         min =          1
     between = 0.2412                                         avg =        2.0
     overall = 0.1842                                         max =          2

                                                Wald chi2(5)      =     256.45
corr(u_i, X)   = 0 (assumed)                    Prob > chi2       =     0.0000

                                   (Std. Err. adjusted for 389 clusters in id)
------------------------------------------------------------------------------
             |               Robust
         fte |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
     treated |
         NJ  |  -.9138251   .9044213    -1.01   0.312    -2.686458    .8588081
        1.bk |   .8437428   1.116946     0.76   0.450    -1.345431    3.032916
       1.kfc |  -9.291282   1.074727    -8.65   0.000    -11.39771   -7.184855
      1.roys |  -1.050834   1.306531    -0.80   0.421    -3.611588    1.509919
    1.wendys |          0  (omitted)
         1.t |  -.1174794   .4526165    -0.26   0.795    -1.004592    .7696327
       _cons |   20.18882   1.178418    17.13   0.000     17.87916    22.49848
-------------+----------------------------------------------------------------
     sigma_u |  5.3011202
     sigma_e |  6.2737008
         rho |  .41656354   (fraction of variance due to u_i)
------------------------------------------------------------------------------

. count if e(sample)==0
  0