Complementarity in Subject Metadata in Large-Scale Digital Libraries: A Comparative Analysis

Provision of high-quality subject metadata is crucial for organizing adequate subject access to rich content aggregated by digital libraries. A number of large-scale digital libraries worldwide are now generating subject metadata to describe not only individual objects but entire digital collections as an integral whole. However, little research to date has been conducted to empirically evaluate the quality of this collection-level subject metadata. The study presented in this article compares free-text and controlled-vocabulary collection-level subject metadata in three large-scale cultural heritage digital libraries in the United States and the European Union. As revealed by this study, the emerging best practices for creating rich collection-level subject metadata includes describing a collection's subject matter with mutually complementary data values in controlled-vocabulary and free-text subject metadata elements. Three kinds of complementarity were observed in this study: one-way complementarity, two-way complementarity, and multiple complementarity.