data SAMPLE (drop = k n);
      retain k 100 n;
      if _n_ = 1 then n = total;
      set POP nobs=total;
      if ranuni(0) <= k / n then do;
         output;
         k = k - 1;
      end;
      n = n - 1;
      if k = 0 then stop;
   run;