这个是把NIANLING分成7个组，为什么我的运行结果总是出错

data sick;

proc freq;

table nianling;

proc capability;

histogram nianling;

run;

data s1;

set sick;

if nianling<15 then fenzu=1;

if nianling>=15 and a<25 then fenzu=2;

if  nianling>=25 and a<35 then fenzu=3;

if  nianling>=35 and a<45 then fenzu=4;

if  nianling>=45 and a<55 then fenzu=5;

if  nianling>=55 and a<65 then fenzu=6;

if  nianling>=65    then fenzu=7;

run;

现实没有找到“NINGLING”