Frequencies for first 10,000 rows of usetobac variable in nhamcsopd2011 dataset :

Tobacco use |      Freq.     Percent        Cum.
------------+-----------------------------------
      Blank |        426        4.26        4.26
    Unknown |      3,151       31.51       35.77
Not current |      5,247       52.47       88.24
    Current |      1,176       11.76      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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