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The NLM Indexing Initiative: Current Status and Role in Improving Access to Biomedical Information
For more than 150 years, the US National Library of Medicine (NLM) has provided access to the biomedical literature through the analytical efforts of human indexers. Since 1966, access has been provided in the form of electronically searchable document surrogates consisting of bibliographic citations, descriptors assigned by indexers from the Medical Subject Headings (MeSH) controlled vocabulary (MeSH, 2012) and, since 1974, author abstracts for many citations.
The MEDLINE/PubMed database contains over 21 million citations. It currently grows at the rate of about 700,000 citations per year and covers 5,591 international biomedical journals in 58 languages. Human indexing consists of reviewing the full text of each article, rather than an abstract or summary, and assigning descriptors that represent the central concepts as well as every other topic that is discussed to a significant extent. Indexers assign descriptors from the MeSH vocabulary of 26,581 main headings, which are often referred to as MeSH Headings (MHs). Main heading descriptors may be further qualified by selections from a collection of 83 topical Subheadings (SHs). In addition there are 203,658 Supplementary Concepts (formerly Supplementary Chemicals) which are available for inclusion in MEDLINE records.
Since 1990, there has been a steady and sizeable increase in the number of articles indexed for MEDLINE, because of both an increase in the number of indexed journals and, to a lesser extent, an increase in the number of in-scope articles in journals that are already being indexed. NLM expects to index over one million articles annually within a few years.
In the face of a growing workload and dwindling resources, we have undertaken the NLM Indexing Initiative (II) to explore indexing methodologies that can help ensure that MEDLINE and other NLM document collections maintain their quality and currency and thereby contribute to NLM’s mission of maintaining quality access to the biomedical literature.