Joseph Yeates starts a two-parter:
This is a technique that I have used in reports that analyze a feature in a data set that is at the bottom of a hierarchy. For example, customer that belong to a larger customer segment or accounts that belong to a grouping. The report analyzes information for an individual customer or account; however, we want to bring in some comparisons for other customers or accounts that belong to the same segment, grouping or cluster.
Click through for the technique and stay tuned for part two.