Hi,
From below dataset, I want to populate previous date for unique subjid. I use below sas code but it is not working. can you please help me.
I need to populate
1) prev_date as separate column
2)A flag1 column that populate 1 when difference between first visit date and last visit date >25.
2)A flag2 column that populate 1 when difference between first visit and previous visit date >25.
data demon;
input subjid $ visitdate : yymmdd10.;
format visitdate yymmdd10.;
datalines;
111 2024-07-15
111 2024-07-22
111 2024-08-29
222 2024-07-15
222 2024-07-22
333 2024-07-24
333 2024-08-15
333 2024-09-25
;
run;
proc sort data=demon;by subjid visitdate;run;
data demo1;
set demon;
by subjid visitdate;
retain prev_date;
if first.subjid then do;
prev_date = .;
end;
else prev_date = lag(visitdate);
format prev_date yymmdd10.;
run;
LAG will only populate its queue when it is called, so you must restructure your IF:
prev_date = lag(visitdate);
if first.subjid then prev_date = .;
You do not lag the first obs.
Try this:
data demon;
input subjid $ visitdate : yymmdd10.;
format visitdate yymmdd10.;
datalines;
111 2024-07-15
111 2024-07-22
111 2024-08-29
222 2024-07-15
222 2024-07-22
333 2024-07-24
333 2024-08-15
333 2024-09-25
;
run;
proc sort data=demon;by subjid visitdate;run;
data demo1;
set demon;
by subjid visitdate;
retain prev_date;
prev_date = lag(visitdate);
if first.subjid then do;
prev_date = .;
end;
format prev_date yymmdd10.;
run;
If you want the flags, you can do this
data demon;
input subjid $ visitdate : yymmdd10.;
format visitdate yymmdd10.;
datalines;
111 2024-07-15
111 2024-07-22
111 2024-08-29
222 2024-07-15
222 2024-07-22
333 2024-07-24
333 2024-08-15
333 2024-09-25
;
run;
proc sort data=demon;by subjid visitdate;run;
data demo1;
set demon;
by subjid visitdate;
retain prev_date;
if first.subjid then do;
prev_date = visitdate;
end;
if not first.id then do;
if visitdate-lag(visitdate)>25 then flag2=1;;
end;
if last.subjid then do;
if visitdate-prev_date>25 then flag1=1;
end;
format prev_date yymmdd10.;
run;
And since you set prev_date in every observation, the RETAIN is not necessary.
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