Classification and categorization: a difference that makes a difference

Information systems are identified as precoordinate when the categories or classes that comprise the system are either assigned or built by the indexer at the time of indexing. A classification system is obviously a precoordinate system because its classes are either established by the classificationist during scheme generation or built by the classifier at the time of class assignment using a faceted vocabulary and a fixed citation order. A subject heading system is also a precoordinate system but it is generally less constrained--and less constraining--than a classification system. Whereas classification mandates assignment of a resource to one and only one class, a precoordinate system of subject headings does not require individual groups to be mutually exclusive. Rather, subject heading systems allow for the assignment of multiple descriptors to a single resource, thereby providing multiple access points for each entity rather than the single access point (the unique class label) prescribed by a classification system.

Because it does not demand a well-defined and absolute relationship between a resource and a subject heading--because it does not require that the groups of entities associated with individual subject headings will necessarily be mutually exclusive--a precoordinate subject heading system is, in fact, a system of categorization. Categories formed by the subject heading system are not rigidly bounded but frequently overlap, with individual members spilling over into penumbral and even alien categories. Although allowing multiple descriptors for a single resource provides for greater variability in the range of resources that can be retrieved with a single query, the questions that can be posed to the information system are nonetheless limited, as they are in a system of classification, by the authorized set of subject heading strings that comprise the system. And, as with a classification system, the retrieval set generated in response to a query is determined by the indexer: the assignment of subject headings as descriptors not only constrains the questions that can be posed to the system but seduces to establish the specific set of resources that can be retrieved in response to each query posed to the system.

Unlike the systematic and principled structure of a classification system, the structure of a subject heading system is frequently unprincipled, unsystematic and polyhierarchical. And, unlike the relationships established between well-defined and mutually exclusive classes in a classification, any relationships created between the categories of a subject heading system cannot be assumed to be either meaningful or information-bearing. An example from Subject Headings for Schools and Public Libraries (Fountain, 2001) illustrates the lack of knowledge-bearing relationships that characterizes many subject heading systems. The heading "Rats as carriers of disease" combines two broader concepts: "rats" and "disease." Although it is obvious that "Rats as carriers of disease" is somehow related to both rats and disease, this heading is neither a kind of "Rat" nor a kind of "Disease." Because the specific value of any relationship that might link this heading to its broader concepts is unidentified, the relationship must be supplied by the individual if the heading is to be linked in a meaningful way to other concepts in the subject heading system.