Frequencies for first 10,000 rows of ultras variable in natlterr2000 dataset :

 Ultrasound |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |      4,018       40.18       40.18
          2 |      5,976       59.76       99.94
          9 |          6        0.06      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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