In reply to Peter Lancashire.

Hi Peter,

The notion of "observation sequence" is intrinsic in many SAS procedures and of course in the DATA step. Since SAS has its roots as statistical software, but later evolved into a powerful data manipulation language, it's not surprising that the concept of "sequence" is so ingrained in the language. Ryan K from Zencos recently posted a nice summary of DATA step vs. SQL.

If you haven't already, you might want to check out PROC DS2, which brings a database-savoir to the concept of data processing.