Libraries must constantly revise and update their collections to ensure that the information they offer is reliable, up-to-date, and valued by their users. Because different types of libraries strive to satisfy different user needs, each library develops its own criteria for evaluating the quality of its collections. Using these evaluations, librarians can determine which areas of the collection most require additional resources. National libraries generally try to ensure that they have at least one copy of every domestically published work as well as a representative selection of foreign works. University and school libraries measure the quality of their existing collections by assessing their ability to support teaching and research at their institutions. These libraries also typically measure the size of their collections or check lists of recommended materials to determine whether the library owns such documents. Public libraries usually judge the quality of their existing collections by determining how often users borrow certain materials. Once librarians assess the quality of their existing collections, they must select and acquire new library materials. Most libraries can buy only a small fraction of the millions of books, periodicals, CD-ROMs, audio and video recordings, and other materials published every year. Large libraries may hire librarians who specialize in the selection of library materials, a process known as collection development. In smaller libraries, senior librarians and other staff members may share responsibilities for developing the library's collection. Unlike national libraries, which typically select all domestic works for their collections, university and school libraries generally select only those works judged by librarians and faculty members to have genuine literary or scholarly merit. Because public librarians must select materials to satisfy the demands of the general reading public, they base their selections almost entirely on readership, circulation, and anticipated demand rather than on the perceived quality of the work. The librarians who select materials for any type of library must balance the needs of their particular clientele with budgetary constraints, space limitations, and other considerations. No one library can own everything needed to satisfy its clientele. Therefore, libraries increasingly emphasize access to materials rather than ownership of materials. For example, libraries may arrange to provide users with access to materials held by other libraries. This cooperation between institutions allows libraries to collectively offer much more comprehensive collections than any one library could offer by itself. Libraries also increasingly provide access to electronic versions of materials instead of stocking the physical materials on the library shelves. Libraries acquire their materials from a variety of sources. The United States Government Printing Office provides libraries in the United States with materials published by the U.S. Congress and by various government departments and agencies. Many libraries-particularly special libraries with rare-book and historical collections-acquire a significant number of their materials as gifts from various organizations, foundations, and individuals. Large libraries often administer approval plans, in which booksellers select materials and send them to libraries in subject areas of interest. The libraries pay for the materials they want to add to their collections and return whatever they do not need. When libraries purchase their materials, they often negotiate discounts from publishers, book dealers, and others in order to stretch the library's budget as far as possible. At some point, all types of libraries must remove older, irrelevant, or infrequently used materials to make space for new acquisitions. This process is sometimes referred to as weeding the collection. Libraries with sufficient budgets may store the removed materials in warehouses or other facilities where they can be accessed on demand. Frequently, however, libraries donate their removed collections to other institutions or sell them in public book sales. In some cases, libraries must destroy these materials.
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