Dear Stata Community

In my paper, I want to investigate the impact of import competition on the incidence of zombie firms. The variables import competition (=penetration) and share of zombie firms (=share_zombiesBH1) are industry-based variables. The variable sic defines in which sector the firm is. Gvkey is the unique identifier for each firm. and at is the number of total assets by each firm.

The datatable looks as follows:

Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input long gvkey float sic double year float(penetration share_zombiesBH1) double at
 1934 20 1989  .03984093 .030927835   184.433
10271 20 1989  .03984093 .030927835   133.559
 8935 20 1989  .03984093 .030927835    4381.7
 5824 20 1989  .03984093 .030927835    3717.6
 4054 20 1989  .03984093 .030927835    17.268
15131 20 1989  .03984093 .030927835     2.884
 1369 20 1989  .03984093 .030927835   139.793
15090 20 1989  .03984093 .030927835     1.486
11748 20 1989  .03984093 .030927835    95.965
 1498 20 1989  .03984093 .030927835   423.518
 5848 20 1989  .03984093 .030927835  1352.919
13324 20 1989  .03984093 .030927835    21.304
 5597 20 1989  .03984093 .030927835  1814.101
 6544 20 1989  .03984093 .030927835      2946
 8852 20 1989  .03984093 .030927835    3221.9
 4809 20 1989  .03984093 .030927835   448.037
13641 20 1989  .03984093 .030927835     7.981
13607 20 1989  .03984093 .030927835     1.987
 1722 20 1989  .03984093 .030927835  4728.308
 7507 20 1989  .03984093 .030927835  1841.913
12825 20 1989  .03984093 .030927835    66.594
 6102 20 1989  .03984093 .030927835   844.339
 2663 20 1989  .03984093 .030927835    3932.1
 9303 20 1989  .03984093 .030927835   111.086
 3138 20 1989  .03984093 .030927835   448.532
 2909 20 1989  .03984093 .030927835    40.322
 3362 20 1989  .03984093 .030927835  4804.161
 5568 20 1989  .03984093 .030927835  4487.451
 1729 20 1989  .03984093 .030927835     4.372
 7770 20 1989  .03984093 .030927835   380.202
13318 20 1989  .03984093 .030927835   186.896
12409 20 1989  .03984093 .030927835   289.361
10793 20 1989  .03984093 .030927835   2586.08
12785 20 1989  .03984093 .030927835   291.102
10177 20 1989  .03984093 .030927835     9.145
14356 20 1989  .03984093 .030927835    42.618
 5141 20 1989  .03984093 .030927835   754.733
13323 20 1989  .03984093 .030927835    81.168
12736 20 1989  .03984093 .030927835     2.685
14891 20 1989  .03984093 .030927835   104.623
13592 20 1989  .03984093 .030927835   115.337
 7146 20 1989  .03984093 .030927835   864.511
 5185 20 1989  .03984093 .030927835   191.928
13864 20 1989  .03984093 .030927835     6.809
 5599 20 1989  .03984093 .030927835   171.749
 8479 20 1989  .03984093 .030927835   15126.7
14273 20 1989  .03984093 .030927835   243.038
 1462 20 1989  .03984093 .030927835   178.954
12201 20 1989  .03984093 .030927835   161.111
 4078 20 1989  .03984093 .030927835   139.408
10899 20 1989  .03984093 .030927835    111.94
14070 20 1989  .03984093 .030927835   329.232
 6375 20 1989  .03984093 .030927835    3390.4
15000 20 1989  .03984093 .030927835    51.464
 9433 20 1989  .03984093 .030927835   481.846
11791 20 1989  .03984093 .030927835    39.347
20338 20 1989  .03984093 .030927835    34.025
 3013 20 1989  .03984093 .030927835   185.989
11424 20 1989  .03984093 .030927835    47.818
 2606 20 1989  .03984093 .030927835    47.165
12309 20 1989  .03984093 .030927835   132.147
14455 20 1989  .03984093 .030927835   213.396
12756 20 1989  .03984093 .030927835  4731.946
 9774 20 1989  .03984093 .030927835   164.886
13930 20 1989  .03984093 .030927835     15.11
 1408 20 1989  .03984093 .030927835   11394.2
 6340 20 1989  .03984093 .030927835    13.616
13136 20 1989  .03984093 .030927835   109.704
10345 20 1989  .03984093 .030927835   113.399
14382 20 1989  .03984093 .030927835     3.464
 2435 20 1989  .03984093 .030927835  1020.984
 8336 20 1989  .03984093 .030927835    14.301
 4050 20 1989  .03984093 .030927835    461.52
 2710 20 1989  .03984093 .030927835   139.293
14332 20 1989  .03984093 .030927835   749.157
11902 20 1989  .03984093 .030927835    28.139
 2674 20 1989  .03984093 .030927835   382.507
 3245 20 1989  .03984093 .030927835    25.191
14057 20 1989  .03984093 .030927835   194.622
 2562 20 1989  .03984093 .030927835    3704.7
 5709 20 1989  .03984093 .030927835   727.429
11713 20 1989  .03984093 .030927835    57.376
19437 20 1989  .03984093 .030927835     1.733
 3144 20 1989  .03984093 .030927835  8282.536
 9411 20 1989  .03984093 .030927835  6522.732
 2675 20 1989  .03984093 .030927835   781.051
13830 20 1989  .03984093 .030927835     8.531
13063 20 1989  .03984093 .030927835    17.732
12614 20 1989  .03984093 .030927835     70.89
15087 20 1989  .03984093 .030927835     1.894
 8582 20 1989  .03984093 .030927835   193.591
 3657 20 1989  .03984093 .030927835   479.687
 1663 20 1989  .03984093 .030927835    9025.7
 2597 20 1989  .03984093 .030927835  3434.042
12566 20 1989  .03984093 .030927835   135.253
 3821 20 1989  .03984093 .030927835   744.759
10551 20 1989  .03984093 .030927835   116.692
11790 21 1989 .003395724          0   432.161
 3642 21 1989 .003395724          0   392.816
 1932 21 1989 .003395724          0 18655.548
end
If I now run the regression, I get the following output. As it can be seen in the output, Stata assumes that the variable gvkey is the group variable, whereas sic should be the group variable. How can I solve this problem?

Code:
 xtreg share_zombiesBH1 penetration at, fe

Fixed-effects (within) regression               Number of obs     =     39,091
Group variable: gvkey                           Number of groups  =      4,927

R-sq:                                           Obs per group:
     within  = 0.1767                                         min =          1
     between = 0.1245                                         avg =        7.9
     overall = 0.1052                                         max =         23

                                                F(2,34162)        =    3667.10
corr(u_i, Xb)  = -0.5759                        Prob > F          =     0.0000

------------------------------------------------------------------------------
share_zomb~1 |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
 penetration |   .4333692   .0052005    83.33   0.000     .4231761    .4435624
          at |   1.87e-07   2.61e-08     7.14   0.000     1.35e-07    2.38e-07
       _cons |   .0014647   .0009884     1.48   0.138    -.0004727     .003402
-------------+----------------------------------------------------------------
     sigma_u |  .05474494
     sigma_e |  .03663516
         rho |  .69069127   (fraction of variance due to u_i)
------------------------------------------------------------------------------
F test that all u_i=0: F(4926, 34162) = 10.17                Prob > F = 0.0000
Many thanks for your help.
Roman