Step-by-Step Programming with Base SAS 9.4

Choosing between the SET Statement and the APPEND Procedure

If two data sets contain the same variables and the variables have the same attributes, then the file that results from concatenating them with the SET statement is the same as the file that results from concatenating them with the APPEND procedure. The APPEND procedure concatenates much faster than the SET statement, particularly when the BASE= data set is large, because the APPEND procedure does not process the observations from the BASE= data set. However, the two methods of concatenating are sufficiently different when the variables or their attributes differ between data sets. In this case, you must consider the differences in behavior before you decide which method to use.