Frequencies for first 10,000 rows of ultras variable in natl1991 dataset :

 Ultrasound |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |      7,488       74.88       74.88
          2 |      2,500       25.00       99.88
          9 |         12        0.12      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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