/*create simulated dataset with 3 patients, A, B, and C*/
data ds1;
     do patient='A','B','C';
 do month=1 to 7;
num=int(ranuni(0)\*10);
output;
end;
end;
run;
/*sort dataset by level of summarization*/
proc sort;
by patient, month;
run;
/*set the lookback period or number of months to lag by*/
%let n = 4;
/*compute statistic of interest*/
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);
/*here we compute a moving average, but this can be any statistic*/
if count ge &n then mov_aver=num_sum/&n;
else mov_aver=.;
run;
/*print results*/title 'Moving average within BY-Group';
proc print;
run;
​