/* Compute the moving average within a BY-group of last n observations.
  For the first n-1 observations within the BY-group, the moving average
  is set to missing. */
data ds1;
do patient='A','B','C';
do month=1 to 7;
  num=int(ranuni(0)*10);
  output;
end;
end;
run;
proc sort;
by patient;
%let n = 4;
data ds2;
set ds1;
by patient;
retain num_sum 0;
if first.patient then do;
  count=0;
  num_sum=0;
end;
count+1;
last&n=lag&n(num);
if count gt &n then num_sum=sum(num_sum,num,-last&n);
else num_sum=sum(num_sum,num);
if count ge &n then mov_aver=num_sum/&n;
else mov_aver=.;
run;

proc print;
run;

不知道红色那句是什么意思呢？

根据这个程序，能不能改编成moving standard deviation of last n observation.觉得貌似可以，但是好像又没那么简单。不知大家有何见解？