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Most of what we have said about the Index relation (e.g., as
part of the vector space model) assumes that keywords are simply a set
of features. But beyond simply providing access to the retrieval of
documents, the fact that keywords are \rikmeaning-ful objects in their
own right means that we can analyze relationships among keywords
directly.
THESAURI are structured representations of relations
among keywords. Common relations represented in thesauri include:
\item
\textbf{Broader term/narrower term (BT/NT)}; these capture hierarchic
relations, generally between a kind of semantics, sometimes part whole
relation. \item \textbf{Related term (RT)} capturing synonym or
quasi-synonym relationships. \item \textbf{Use for (UF)} capturing a
preferred, conventional or authoritative term over possible
alternatives.
One of the most extensive examples of such a
representation is the MESH (MEdical Subject Headings) thesaurus, part of
the National Library of Medicine's extensive PubMed system. FIgure
(figure) shows the term {\tt LYMPHOMA} within the MESH
thesaurus. expanded in the second two tree locations.} example
Hierarchic BT/NT relations are shown as indentation. Because this
thesaurus allows a single keyword to fit in multiple places in the
hierarchy (e.g., treating {\tt LYMPHOMA} as a kind of {\tt NEOPLASM} as
well as a kind of {\tt IMMUNOLOGIC DISEASE} ) this browser shows the
term as part of three separate paths; note that the children (narrower
terms) of {\tt LYMPHOMA} are repeated at each location.
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Keyword structures
Subsections