Inter-Indexer Consistency

Inter-indexer consistency measures how far two or more people agree when they independently assign subject terms to the same documents. Because subject indexing is a judgment task — choosing which descriptors best represent a document's content — different indexers routinely pick overlapping but not identical term sets, and the degree of that overlap is a fundamental indicator of the reliability of an indexing system. The standard quantity is the Hooper-style consistency coefficient, the size of the shared term set divided by the size of the combined term set, averaged across documents; Zunde and Dexter and later Rolling refined it and connected it to indexing quality. Low consistency signals that retrieval will be unpredictable, since whether a document is found can depend on which indexer happened to process it.