How do I use PROC EXPAND to fill in time series observations within a panel (longitudinal) data set? - sas

Question

I'm using this SAS code:
data test1;
input cust_id $
month
category $
status $;
datalines;
A 200003 ABC C
A 200004 DEF C
A 200006 XYZ 3
B 199910 ASD X
B 199912 ASD C
;
quit;
proc sql;
create view test2 as
select cust_id, input(put(month, 6.), yymmn6.) as month format date9.,
category, status from test1 order by cust_id, month asc;
quit;
proc expand data=test2 out=test3 to=month method=none;
by cust_id;
id month;
quit;
proc print data=test3;
title "after expand";
quit;
and I want to create a dataset that looks like this:
Obs cust_id month category status
1 A 01MAR2000 ABC C
2 A 01APR2000 DEF C
3 A 01MAY2000 . .
4 A 01JUN2000 XYZ 3
5 B 01OCT1999 ASD X
6 B 01NOV1999 . .
7 B 01DEC1999 ASD C
but the output from proc expand just says "Nothing to do. The data set WORK.TEST3 has 0 observations and 0 variables." I don't want/need to change the frequency of the data, just interpolate it with missing values.
What am I doing wrong here? I think proc expand is the correct procedure to use, based on this example and the documentation, but for whatever reason it doesn't create the data.

Answer 1

You need to add a VAR statement. Unfortunately, the variables need to be numeric. So just expand the month by cust_id. Then join back the original values.
proc expand data=test2 out=test3 to=month ;
by cust_id;
id month;
var _numeric_;
quit;
proc sql noprint;
create table test4 as
select a.*,
b.category,
b.status
from test3 as a
left join
test2 as b
on a.cust_id = b.cust_id
and a.month = b.month;
quit;
proc print data=test4;
title "after expand";
quit;