Selection of MEDLINE contents, the development of its thesaurus, and the indexing process.

The operation of MEDLINE requires three ongoing activities by persons having subject matter knowledge. These are literature selection, thesaurus maintenance and indexing. MEDLINE is intended to give access to the most generally useful biomedical literature rather than to provide indiscriminate comprehensive coverage. Literature is selected with the guidance of a group of health-science educators, editors and librarians who review periodicals under consideration for inclusion and re-evaluate those that are already regularly indexed. The MeSH thesaurus provides the descriptors that are used for subject indexing. Its hierarchical structure facilitates both general and specific searching. The appearance of new concepts and terminology in the literature requires a dynamic MeSH, but MeSH changes may complicate the process of searching backward in time. Maintaining MeSH requires finding a balance between the need for adaptability and the need for stability. Quality indexing requires accuracy and consistency in the assignment of subject headings. To this end, indexers receive didactic training plus practice under supervision. Precedents for indexers are detailed in an extensive Indexer's Manual and in MeSH annotations. Work of all indexers is reviewed on at least a sampling basis, and special sessions are held each year to familiarize indexers with MeSH changes.