Frequencies for first 10,000 rows of popsum variable in nhamcsopd1993 dataset :

# of clinic |
   sampling |
   units in |
   hospital |      Freq.     Percent        Cum.
------------+-----------------------------------
         -1 |      7,170       71.70       71.70
          0 |      2,830       28.30      100.00
------------+-----------------------------------
      Total |     10,000      100.00

by Jean Roth , jroth@nber.org , 28 Mar 2016