案例
从A，B组各抽25%条出来，抽出的观察值放数据集ex1中，剩下的放ex2中

proc sort ;
by group;
run;
proc surveyselect data=ex rate=0.25   outall  out=ex;
    strata group ;
   run;
data ex1 ex2;
set ex;
if selected then output ex1; else output ex2;
keep  group var1-var4;
run;