Frequencies for first 10,000 rows of nofollow variable in nhamcsed1999 dataset :

         no |
  follow-up |
    planned |      Freq.     Percent        Cum.
------------+-----------------------------------
          0 |      9,250       92.50       92.50
          1 |        750        7.50      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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