Frequencies for prev_dwi variable in vehicle dataset :

   PREVIOUS |
        DWI |      Freq.     Percent        Cum.
------------+-----------------------------------
          0 |        567       90.58       90.58
         99 |         58        9.27       99.84
          . |          1        0.16      100.00
------------+-----------------------------------
      Total |        626      100.00

by Jean Roth , jroth@nber.org , 18 Apr 2016