Controlled Vocabulary Indexing

Controlled vocabulary indexing is the process of representing what a document is about by assigning preferred terms drawn from an established controlled vocabulary or thesaurus, rather than from the document's own free-text words. ANSI/NISO Z39.19 codifies the practice: the indexer first performs conceptual analysis to determine a document's aboutness, then translates each concept into the vocabulary's preferred term, choosing how many concepts to capture (exhaustivity) and how finely to express each (specificity). Elaine Svenonius's account of subject languages explains why this controlled translation matters — it eliminates the synonymy and homonymy of natural language so that one concept is always indexed under one term. Done consistently, controlled vocabulary indexing gives a collection reliable, predictable subject access that free-text search alone cannot guarantee.