As demands on libraries continue to grow, outpacing budget increases, more librarians are forced to make difficult decisions about what materials and services stay and go. Charles R. McClure has written that many librarians use an "adhocracy" method to make these decisions, relying on no data or simple aggregates in determining a course of action. Others, however, have turned to a more data-driven approach that moves beyond generalized aggregations, such as running totals and overall means, to reveal underlying patterns in the data that clarify which services or materials are worth retaining. In this article, the author discusses "data warehousing" techniques that explore a large dataset for nontrivial, novel, and useful patterns, using different statistical, analytical, and visualization tools. (Contains 1 figure.)