sas 求和

data test;
infile cards dlm=',';
input site $ year month value;
cards;
51701,1951,1,10,
51701,1951,2,20,
51701,1951,3,30,
51701,1951,4,40,
51701,1951,5,50,
51701,1951,6,60,
51701,1951,7,70,
51701,1951,8,80,
51701,1951,9,90,
51701,1951,10,10,
51701,1951,11,20,
51701,1951,12,30,
51701,1952,1,10,
51701,1952,2,20,
51701,1952,3,30,
51701,1952,4,40,
51701,1952,5,50,
51701,1952,6,60,
51701,1952,7,70,
51701,1952,8,80,
51701,1952,9,90,
51701,1952,10,10,
51701,1952,11,20,
51701,1952,12,30,
51701,1953,1,10,
51701,1953,2,20,
51701,1953,3,30,
51701,1953,4,40,
51701,1953,5,50,
51701,1953,6,60,
51701,1953,7,70,
51701,1953,8,80,
51701,1953,9,90,
51701,1953,10,10,
51701,1953,11,20,
51701,1953,12,30
;
run;
*第一年9月份以后到第二年3月份的数据，我理解是51年10月-52年3月，52年10月-53年3月...
*时间段位6个月，要求它们value的和，可以先求和再筛选。
*我的方法是利用lag函数，进行倒推，依次是value,lag(value)，....,然后求和
*最后进行月份筛选需要的和，这里是从每年3月倒推至上一年的10月
;
data need ;
set test;
val_sum = value + lag(value)+ lag2(value)
+ lag3(value) + lag4(value) + lag5(value);
if month = 3 then output;
run;