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

   national |
    highway |
     system |      Freq.     Percent        Cum.
------------+-----------------------------------
          0 |      6,595       65.95       65.95
          1 |      3,397       33.97       99.92
          9 |          8        0.08      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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