Frequencies for first 10,000 rows of preterm variable in natl1990 dataset :

   Previous |
 preterm or |
small-for-g |
estational- |
 age infant |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |         66        0.66        0.66
          2 |      9,885       98.85       99.51
          8 |          1        0.01       99.52
          9 |         48        0.48      100.00
------------+-----------------------------------
      Total |     10,000      100.00

by Jean Roth , jroth@nber.org , 3 Jan 2016