creating a longterm average

[<linton.corbet@xxxxxxxx>]

Hi!
I am wondering what is the easiest way to create a long term
average (of say, sst from 1970-1980), and then be able to compare 
it to another longterm average?

Is usually done by creating a virtual variable?

Any help/suggestions greatly appreciated.

Cheers 
L. Corbet

(P.S. using LAS version 6.0)