Frequencies for first 10,000 rows of preterm variable in natl1996 dataset :

   Previous |
 preterm or |
small-for-g |
estational- |
 age infant |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |         84        0.84        0.84
          2 |      9,888       98.88       99.72
          9 |         28        0.28      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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