Testing is one of direct marketing's great differentiators. Although you can test many other things—media usage, offers, creative approaches, even types of postage—list testing is generally an experienced direct marketer's first priority. Basically, list testing means selecting a test quantity from several different lists, coding each list for tracking purposes, and evaluating the results with the intent of reusing some lists, adding new lists, and potentially eliminating some lists. Again, this is something you should not "fool around" with a direct marketing agency or experienced list broker can be a valuable ally when it comes to list testing.