/*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;
​