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Faceted classification and logical division in information retrieval

Library Trends, Wntr, 2004 by Jack Mills

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7.1. Division must be exhaustive

The constituent species collectively must be coextensive with the extension of the genus. The obvious difficulty encountered here is that of our imperfect knowledge. This can be overcome in a technical sense by the process of dichotomy, in which one species is named and all the others are covered by its negative, e.g., the array (Buildings by material) could give just two classes, brick buildings and nonbrick buildings, and this would exhaust the array--no buildings would be missed. In practice, of course, all significant kinds of other materials would be enumerated with a possible residual class for "Others."

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7.2. Each step of division should be proximate

Division should not make a leap. Like exhaustivity, this is a counsel of perfection, which in practice is limited by imperfections in our knowledge. The price of failure is the obscuring of relations that might in fact be important in the definition of classes. Division of transport systems into road, rail, sea, and air obscures the relationship of road and rail as being kinds of land transport and of sea transport being a kind of water transport. In this example, more than one characteristic of division has been overlooked, e.g., land and water represent division by the characteristic of natural medium, but road and rail reflect the characteristic of form of track, which is special to land transport.

7.3. Special problems of division into arrays

As a faceted classification moves into more and more detailed analysis of a subject, more and more arrays are disclosed and some of these pose special problems. Several examples have been given already of the situation in which terms appearing in one facet (as properties, materials, parts, etc.) appear also in other facets in a different relationship. For example, the Materials facet in Building technology includes timber; this could qualify a structural unit (e.g., timber for fencing). But it also could define a unit as being a kind of structure (e.g., timber houses). This relation is called specification (species-making). BC2 now generalizes this situation by assuming the possibility of terms from any facet behaving in this way, and this may be seen as a particular example of the general theory of analytico-synthetic classification. The distinction between qualification and specification was regarded by Metcalfe (1957) as a major feature of the relations found in indexing. At the most general level, it reflects the distinction between the inclusion relation (generic, semantic, hierarchical) and syntactic relations (see Section 12.3). It poses a particular problem in the entity (end-product, purpose) facet (see Section 8.3) but can appear in other facets, e.g., the concept of prefabricated bathrooms (those fabricated off-site and installed in toto in different kinds of buildings) reflects a part of a building (a room) specified by an operation (prefabrication). In BC2, wherever the need is demonstrated, the array reflecting the primary entity in a subject (e.g., in Building technology, the Buildings by function array) is preceded by a number of arrays derived by specification using other facets, for example, Buildings by detachment, Buildings by number of stories. In chemistry, the primary entity array (Substances by chemical constitution--i.e., elements and their compounds) is preceded by a number of arrays defined by concepts from other facets (Behavioral properties, Structural properties) and so on. In nearly all classes these other, derivative arrays appear in the same order as their defining facets appear in the class in general. In this respect, it has been noted (Coates, 1973) that a faceted classification provides a potent medium whereby newly emergent classes can be accommodated in a consistent and predictable fashion.

Faceted classification and logical division in information retrieval

Library Trends, Wntr, 2004 by Jack Mills

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