DUPLICATION IN UNION CATALOGUES

This paper uses figures of the overlap between eighteen libraries to develop a mathematical model of the duplication likely in a union catalogue. Two consequencescome from the model, that the percentage of unique items held by a library depends on the type of the library rather than on its size, and that for a homogeneous group of libraries the proportion of new titles added when extra entries are made to the catalogue tends to a value greater than zero. These consequences, which are supported by further data, suggest that the law of diminishing returns is much weaker than might be expected when building up a union catalogue.