DATA test;
input Week $ Day Price;
CARDS;
111 1 1
111 1 2
112 1 3
112 1 4
111 2 5
111 2 6
112 2 7
112 2 8
;
run;
proc sql;
create table means as
select distinct *, mean(price) as mean_price
from test
group by week, day;
quit;

以上程序是按照 week, day分组得到平均值，但是把mean(price)  换成std(price)后，不能得到分组后的标准差，
怎么办，请大神指教！！！