请问为什么 第5个observation level的值是medium？谢谢。
data levels;
input obs name $ level;
cards;
1 Frank 1
2 Joan 2
3 Sui 2
4 Jose 3
5 Burt 55
6 Kelly .
7 Juan 1
;
run;
data expertise;
set levels;
if level=. then  expertise ='unknown';
   else if level=1       then expertise= 'low';
   else if level =2 or 3 then expertise='Medium';
   else expertise='high';
run;
proc print data=expertise;
run;