Frequencies for first 10,000 rows of op_cerv variable in natl2013 dataset :

   Cervical |
   Cerclage |
        RFP |
601=f_ob_ce |
     rvic R |      Freq.     Percent        Cum.
------------+-----------------------------------
          N |      9,747       97.47       97.47
          U |        213        2.13       99.60
          Y |         40        0.40      100.00
------------+-----------------------------------
      Total |     10,000      100.00

by Jean Roth , jroth@nber.org , 22 Dec 2015