Frequencies for first 10,000 rows of nhs variable in accident dataset :

   national |
    highway |
     system |      Freq.     Percent        Cum.
------------+-----------------------------------
          0 |      5,147       51.47       51.47
          1 |      3,569       35.69       87.16
          9 |      1,284       12.84      100.00
------------+-----------------------------------
      Total |     10,000      100.00

by Jean Roth , jroth@nber.org , 22 Oct 2016