With the addition of one more variable, publication date, it is possible to analyze collection growth. All librarians think of collection size as a variable in assessment, but not all realize that growth can be articulated as a by-product of size. The degree of difficulty in providing this statistic will depend on the library's management system and history. At most libraries, order records in the acquisitions module are erased at some point in time. Thus, the received date in the order record cannot be used as a count of yearly acquisitions. Likewise, virtually all libraries have not only changed from card to computerized systems, but many have switched from one computerized system to another over time. Quite frequently it is the case that information on original received dates did not migrate from one to another system. Although "growth" subsequently becomes a slight misnomer, the solution is to treat publication dates as acquisition dates.

When publication dates are used to represent acquisition dates, it is possible to track growth by gathering statistics within a certain call number range with publication dates prior to, for example, 1995; repeat this same process with publication dates up to 2005; and report the percentage of change or growth over that decade. Various combinations of years can be utilized for comparisons, e.g., to demonstrate the percentage of change within the last two years as compared with the two prior, or to track growth over time from 1950s to the present.

The weakness of a methodology that relies on publication date as an acquisition date is that it will not record retrospective buying. It is therefore important know the history of the library and its collections. It is nonsensical, for example, to treat books with 1880 publication dates as acquisitions in the 19th century if the library itself is only fifty years old. It is also imperative to have knowledge of the library's collecting practices. For example, it is not uncommon to institute a retrospective buying project for the purpose of building up a weak, new or small collection. If, between 2000 and 2001, the selector for the new South East Asian collection was given extra funds to concentrate more on purchasing older titles (published in the 1970s) than contemporary titles (2000s), the data could easily be misinterpreted. It would appear as though the collection has been in existence for the last thirty or more years and, depending on the proportions, it may also appear as though the collection had steady growth in the 1970s and a drop in growth in the early 2000s. If retrospective buying has been minimal, however, then there is no reason to fear grave misinterpretation of the data.

Though relative strength and growth are simple statistics, these are powerful statistics, especially when the methodologies are combined together. The measurement of relative subject collection strengths helps to clarify whether or not the emphasis within the collection matches research and teaching in corresponding academic departments. That is important information; however, it is possible that the results demonstrate the effects of past collecting practices and more important information is hidden. By tracking relative strength over time (collection sizes within sub-ranges sorted by publication dates) it is possible to analyze whether or not collecting in the last ten years has been going in the same direction as research interests.

"Growth" can be employed to indicate if particular collections are developing as needed or in some significant way, and to answer these questions:

Has collection focus changed over time? While collection strengths can be measured by looking at total holdings, it is possible that these numbers portray collection development policies that no longer apply. For example, "Constitutional History and Administration" may be the largest subfield collection in overall Political Science collection, but the collection development policy for the last twenty years says "International Law & International Relations" should have the most emphasis. Do the data of the last twenty years increasingly reflect this shift?

Which subject areas have grown the most and which the least in the decades of the last half-century?

How vigorous has the library's collecting been in comparison to peers over the last twenty years?

Are there subjects or subfields that appear to have lost collection emphasis or been overlooked in the last few years?

Data & Methods Bank