I guess this is a simple question.

This is a monthly dataset. Permno is an unique code for firms.

How to screen firms with at least 12 monthly observations?

And how to screen firms with at least 12 consecutive monthly observations?

Here is the example data:


Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input double permno long date double(shrcd exchcd prc ret shrout)
10000  9496  . .         .                     .    .
10000  9527 10 3    -4.375                     . 3680
10000  9555 10 3     -3.25    -.2571428716182709 3680
10000  9586 10 3   -4.4375    .36538460850715637 3680
10000  9616 10 3        -4   -.09859155118465424 3793
10000  9646 10 3 -3.109375            -.22265625 3793
10000  9677 10 3  -3.09375  -.005025125574320555 3793
10000  9708 10 3  -2.84375   -.08080808073282242 3793
10000  9737 10 3  -1.09375    -.6153846383094788 3793
10000  9769 10 3  -1.03125   -.05714285746216774 3793
10000  9800 10 3   -.78125   -.24242424964904785 3843
10000  9828 10 3  -.828125    .05999999865889549 3843
10000  9861 10 3  -.515625   -.37735849618911743 3843
10000  9891 10 3   -.40625   -.21212121844291687 3893
10000  9919 10 3   -.40625                     0 3893
10000  9951 10 3      -.25   -.38461539149284363 3893
10000  9981 10 3  -.234375                -.0625 3893
10000 10010 10 3   -.21875   -.06666667014360428 3893
10000 10042 10 3         .                     . 3893
10001  9496  . .         .                     .    .
10001  9527 11 3    -6.125                     .  985
10001  9555 11 3     -6.25   .020408162847161293  985
10001  9586 11 3   -6.3125   .025200003758072853  985
10001  9616 11 3    -6.375   .009900989942252636  985
10001  9646 11 3   -6.3125  -.009803921915590763  985
10001  9677 11 3    -6.125  -.013069307431578636  985
10001  9708 11 3   -6.0625  -.010204081423580647  985
10001  9737 11 3      -6.5    .07216494530439377  985
10001  9769 11 3     6.375  -.003076923545449972  991
10001  9800 11 3     6.625    .03921568766236305  991
10001  9828 11 3         7   .056603774428367615  991
10001  9861 11 3         7   .014999999664723873  991
10001  9891 11 3      6.75    -.0357142873108387  991
10001  9919 11 3      6.25   -.07407407462596893  991
10001  9951 11 3     6.375    .03680000081658363  991
10001  9981 11 3    -6.125   -.03921568766236305  991
10001 10010 11 3   -5.6875    -.0714285746216774  991
10001 10042 11 3     5.875   .051428571343421936  991
10001 10073 11 3         6   .021276595070958138  991
10001 10104 11 3       6.5     .0833333358168602  991
10001 10134 11 3      6.25   -.02230769209563732  992
10001 10164 11 3     6.375   .019999999552965164  992
10001 10195 11 3    6.1875  -.029411764815449715  992
10001 10226 11 3     5.875   -.03353535383939743  992
10001 10255 11 3      6.25    .06382978707551956  992
10001 10286 11 3      6.75    .07999999821186066  992
10001 10317 11 3     6.125   -.07629629969596863  992
10001 10346 11 3   -6.3125   .030612245202064514  992
10001 10378 11 3   -6.4375   .019801979884505272  992
10001 10408 11 3      6.25   -.01203883532434702  992
10001 10437 11 3   -6.4375   .029999999329447746  992
10001 10470 11 3     6.625   .029126213863492012  992
10001 10500 11 3    -6.375  -.021132076159119606  998
10001 10531 11 3     6.625    .03921568766236305  998
10001 10561 11 3     6.625                     0  998
10001 10591 11 3     6.375  -.021132076159119606  998
10001 10623 11 3       6.5   .019607843831181526  998
10001 10651 11 3      6.75    .03846153989434242  998
10001 10682 11 3      6.75   .017777778208255768  998
10001 10710 11 3      7.25    .07407407462596893  998
10001 10743 11 3         7   -.03448275849223137  998
10001 10773 11 3         7   .017142856493592262 1001
10001 10804 11 3      7.25     .0357142873108387 1001
10001 10835 11 3      9.25    .27586206793785095 1001
10001 10864 11 3     8.875  -.027027027681469917 1019
10001 10896 11 3       9.5    .07042253762483597 1019
10001 10926 11 3     9.875    .03947368264198303 1019
10001 10955 11 3    10.125   .037974681705236435 1022
10001 10988 11 3   -9.9375  -.018518518656492233 1022
10001 11016 11 3    -9.875  -.006289307959377766 1022
10001 11046 11 3    -9.875   .012658228166401386 1027
10001 11077 11 3    -9.875                     0 1027
10001 11108 11 3      9.75  -.012658228166401386 1027
10001 11137 11 3      9.75   .014102564193308353 1031
10001 11169 11 3       -10   .025641025975346565 1031
10001 11200 11 3       9.5   -.05000000074505806 1031
10001 11228 11 3      9.75    .04078947380185127 1044
10001 11261 11 3    -9.625  -.012820512987673283 1044
10001 11291 11 3     9.625                     0 1044
10001 11322 11 3      -9.5    .00129870162345469 1054
10001 11353 11 3     9.625    .01315789483487606 1054
10001 11381 11 3      9.75   .012987012974917889 1054
10001 11409 11 3       9.5  -.011538460850715637 1054
10001 11442 11 3    -9.875    .03947368264198303 1054
10001 11473 11 3    -9.875                     0 1054
10001 11501 11 3      10.5    .07848101109266281 1073
10001 11534 11 3    10.125    -.0357142873108387 1073
10001 11564 11 3      11.5     .1358024626970291 1073
10001 11595 11 3      11.5   .013043479062616825 1073
10001 11626 11 3        13     .1304347813129425 1073
10001 11655 11 3     14.75    .13461539149284363 1073
10001 11687 11 3      14.5 -.0067796604707837105 1075
10001 11718 11 3     13.75    -.0517241396009922 1075
10001 11746 11 3        11   -.20000000298023224 1075
10001 11778 11 3     11.75    .08181818574666977 1075
10001 11808 11 3   -11.875   .010638297535479069 1075
10001 11837 11 3       -12   .010526316240429878 1075
10001 11869 11 3     11.75  -.007708333432674408 1075
10001 11900 11 3      12.5    .06382978707551956 1075
10001 11931 11 3        13    .03999999910593033 1075
end
format %d date
Thanks!