Frequencies for first 10,000 rows of usetobac variable in nhamcsopd2007 dataset :

tobacco use |      Freq.     Percent        Cum.
------------+-----------------------------------
      blank |        522        5.22        5.22
    unknown |      3,865       38.65       43.87
not current |      4,439       44.39       88.26
    current |      1,174       11.74      100.00
------------+-----------------------------------
      Total |     10,000      100.00

by Jean Roth , jroth@nber.org , 28 Mar 2016