Overlap measures the degree of duplication that exists within a set of libraries. At its highest aggregate level (overall total holdings), overlap provides an indicator of the degree to which library collections are "vanilla" or identical.
Such an inquiry may be of great interest, but it may also be deceptive. It is, in particular, the relatively recent rise in use of approval plans that has caused fear of over-duplication. To investigate this phenomenon, rather than examining total holdings, a greater test of whether or not approval plans and other recent structural factors have flavored university collections vanilla can be found by running an overlap analysis limited by dates of publication. Comparisons of the last decades of the 20th century may indicate whether or not a trend toward title duplication has developed, a trend that would not otherwise be evident in a comparison of total holdings overlap.
Overlap analysis can be utilized to answer the following questions:
Which peer institution has the highest rate of duplication with your library? Excel can sort the data into an array.
Chart the rates of overlap for the last ten years with each peer library to see if the rate of duplication is on the rise and to map areas of duplication. In subject areas in which duplication is not desirable, are the rates too high? The results may provide evidence that assigned primary collection responsibilities need to be reconsidered.
Titles that duplicate with those of a peer in any particular subject may be understood as the "core” collection and the rest (those that do not duplicate) may be construed as "sharable." Among the library’s strongest collections, what percent is sharable with your peer? Which subjects have the smallest percent of core titles?
When overlap is examined in terms of the age of collections, are the core collections primarily composed of newer materials? What percent of sharable titles in your strongest collections are recent acquisitions?