Matt Allington solves a particular associative purchasing behavior problem:
The requirement is to be able to select a single customer from a list of customers and see what products they have purchased. Then the next things is to be able to compare sales of this same group of products across all customers while ignoring sales of all other products. Stated another way, the report should show sales from all customers but only for the products purchased by the original selected customer. Now there is a “cheats” solution to this problem using visual level filters – more on that later. The purpose of this article is to show a DAX only solution.
Read Matt’s answer, but also check out the comments for an even better solution if you’re using a proper star schema.