Frequencies for first 10,000 rows of rf_ppterm variable in natl2014 dataset :

   Previous |
    Preterm |
Birth Y Yes |      Freq.     Percent        Cum.
------------+-----------------------------------
          N |      9,413       94.13       94.13
          U |        128        1.28       95.41
          Y |        459        4.59      100.00
------------+-----------------------------------
      Total |     10,000      100.00

by Jean Roth , jroth@nber.org , 15 Jun 2016