Access to biomedical information: the Unified Medical Language System

The possible uses for the information embodied in the Semantic Network are easy to imagine but difficult to realize in an automated environment and await software development for full exploitation. The semantic types should, nevertheless, make it possible for computer systems to organize biomedical concepts effectively and to reason about the possible and probable relationships among different types of concepts (Lindberg & Humphreys, 1989). Such systems may employ linguistic parsing techniques for automated indexing of biomedical literature or automated analysis of clinical data. Use of the Semantic Network may assist in query formulation, interactive query refinement, or simply graphical browsing of the Metathesaurus (McCray & Hole, 1990).

Future editions of the Semantic Network may see additional relationships among existing semantic types and the addition of new semantic types. Adding new semantic types naturally involves possible reassignment of semantic types to Meta concepts, increasing the complexity of the updating program.

Bishop and Ewing (1992) have suggested that the UMLS developers missed an important link between the Metathesaurus and the Semantic Network by not relating Meta unique term identifiers to semantic types in some way. Meta does not use a hierarchical coding scheme, as do most of the vocabularies being merged. Coding of concepts is helpful for establishing consistent recognition of concepts as opposed to the names for concepts (names may change over time and among environments), for recognizing concepts in different languages, and for efficiently maintaining compatibility between systems. Meta uses random coding, that is, a coding that carries no information about how one concept is related to another. Were the coding itself to reflect in some way the hierarchical relationships among terms as represented by the semantic types, users could more easily extract classes of concepts. Bishop and Ewing (1992) further suggest that, even though most existing coding schemes differ stylistically, the hierarchies they are based on are quite similar and could form the basis of an ideal arrangement of medical knowledge for the future. The UMLS developers might have extended the idea of thesaurus integration, with its empirical founding, to the Semantic Network that purports to organize the concepts in them.

The Property of Semantic Locality

Because it functions as a thesaurus, the Metathesaurus is fundamentally a device for organizing meanings. It should do this in a way that permits a user seeking a term for a meaning to find that term by navigating its relationships to other terms. A thesaurus makes this possible by giving to each of its terms a semantic locality. Semantic locality has other uses as well, including establishing what is generally relevant to a given concept, or what may be relevant in a given situation (Nelson et al., 1992, p. 213).

The provision of semantic locality in the UMLS is particularly generous and goes beyond other thesauri. It is provided by semantic types; by term information that includes synonyms, related terms, and lexical variants; by co-occurrence data; and by contextual data, or, a term's parents, siblings, and children derived from the source vocabularies (Nelson et al., 1992, p. 210). Even though the contextual data may differ or even conflict, the differences may reflect the different intentions and viewpoints of the source vocabularies, and may have value as such. All of these elements help to establish meanings, locate more general and more specific terms, and find potentially useful relationships between terms.