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

   national |
    highway |
     system |      Freq.     Percent        Cum.
------------+-----------------------------------
          0 |      6,941       69.41       69.41
          1 |      3,053       30.53       99.94
          9 |          6        0.06      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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