data a;

input y age chage rs lc mr ;
cards;
0 0 0 1 0 0
0 0 0 1 1 0
0 0 0 2 0 0
0 0 0 1 0 0
0 0 0 2 0 0
1 1 1 1 1 0
1 1 1 3 2 1
1 1 1 3 2 1
1 1 1 3 2 1
1 1 1 1 2 1
;
proc logistic data=a  desc plots(only)=(effect(link join=yes));

class rs / param=reference ref=last ;
model y=rs;
run;
为什么当ref=last 与ref=first的结果一样呢？如图