This sounds like an interview question I've seen many times before - interleave uses SET (with or without a BY) and match-merge uses MERGE statement (with a BY statement, to achieve the match).
And, the results are very much different - suggest the OP try it with the SASHELP "sample" dataset members and use various self-diagnostic techniques, such as PROC PRINT and use a PUTLOG _ALL_; statement in the DATA step where using the SET/MERGE statements.
Scott Barry
SBBWorks, Inc.
Suggested Google advanced search arguments, this topic / post (clearly optional and no doubt extra-credit -- notice no links were contributed, nor harmed with this post-reply):
data step programming set merge site:sas.com
data step programming by group processing site:sas.com
Message was edited by: sbb