Adding cases by group in SAS - merge

Question

I have an SAS dataset in a long format. The intervention program started from 2014 Spring semester, and it has been on until 2017 Spring semester. So there has been 7 semesters (2014 Spring and Fall, 2015 Spring and Fall, 2016 Spring and Fall, 2017 Spring).
Not everyone participated in all 7 semesters though. Some participated once and never came back, some participated more than twice but not necessarily two semesters in a low.
So each individual has a different number of cases. For someone who participated twice, for example, has 2 rows, some with 5 participations have 5 rows.
I want everyone has 7 rows in the dataset for some reason.
What could be the best way of programming to do this in SAS?
I would really appreciate any suggestions!

Answer 1

PROC EXPAND is probably the most direct way to do this, although it has the limitation that it won't extrapolate beyond the observed start/end of the range, and it expects regular intervals (or a lot more work to define intervals).
proc expand data=your_data out=expanded_data from=semiyear extrapolate method=none;
by student;
id semester_date;
run;
That relies on semester_date being a date variable corresponding generally to the start of each half-year.
Perhaps more easily in this case, you could use the printmiss option in proc tabulate, which would generate a table pretty easily.
ods output table=out_table;
proc tabulate data=your_data;
class student semester;
tables student,semester/printmiss misstext=' ';
run;
ods output close;
Then merge that back to the main dataset, this will have a row for every student*semester combination.