Does efficiency really matter?  You only need to transform the data once.  Anyway here's one way (assuming your data is sorted by ID): proc transpose data=have prefix=start out=start_dates;    by id;    var start; run; proc transpose data=have prefix=end out=end_dates;    by id;    var end; run; data want;    merge start_dates end_dates;    by id; run; You will find that some programming tasks are easier, but they now depend on you knowing how many start and end dates you have. Good luck. ... View more