Classification and categorization: a difference that makes a difference

By allowing the individual to generate her own queries, the postcoordinate system supports a more interactive form of communication between searcher and system. In most postcoordinate systems, descriptors are assigned from a controlled vocabulary. In others, however, communication between the individual and the information system is complicated by the fact that the indexing language does not exist as a controlled vocabulary but is extracted by the indexer from terms occurring in the resource being indexed. Generally, however, the generation of category definitions as postcoordinate search queries is limited only by the set of individual terms that comprise the indexing language. Although the resources that participate in a retrieval set are determined by the indexer's assignment of descriptors, communication between the system and the individual is greatly enhanced by her ability to create her own queries that will capture her immediate information need.

Unfortunately, however, the flexibility of category generation, like the process of cognitive categorization, goes hand-in-hand with the absence of meaningful relationships. As with any free-text information system, posing a query to a postcoordinate system simply divides the collection into two groups: the set of resources whose assigned descriptors match the search query and the remaining resources whose descriptors do not match the query. Obviously, postcoordinate systems, like free-text systems, are simply mechanisms for grouping, not systems of organization. Unlike free-text systems, however, the basis for grouping in a postcoordinate system is semantic, not syntactic. Although the postcoordinate system is simply matching strings, the indexer imposes a certain level of conceptual control by assigning simple descriptors from an indexing language that establishes an indexical, one-for-one relationship between a descriptor and its referent. The individual is empowered to create unique and potentially idiosyncratic search categories precisely because the system itself does not establish any but the simplest categories--those defined by the individual descriptors assigned by an indexer. Because the system fails to establish a principled framework that provides for the establishment of information-bearing relationships between categories, the postcoordinate system can neither create nor contribute to an information context precisely because there is no persistent structure that could support meaningful relationships between categories.

CONCLUSION

This very preliminary review of the properties and features of the different approaches to organizing, ordering, or simply grouping information resources has barely scratched the surface in addressing structural distinctions between systems of classification and systems of categorization and how these distinctions affect interaction with the system as an information environment.

For example, at a very superficial level, the strength of classification is its ability to establish relationships between classes that are stable and meaningful. But the rigidity of structure that supports these relationships has its corresponding disadvantages. In particular, traditional classification systems are context-independent: because the relationships established by classification are invariant and persist across time and space, these systems are resilient to the context of use and severely constrain the individual's ability to communicate with the system in a meaningful and productive manner. In contrast, systems of categorization, and especially postcoordinate systems, are highly responsive to--even dependent on--the immediate context. The utility of these systems as information environments depends ultimately on provisions for effective communication with the individual. But the responsiveness and flexibility of the postcoordinate system effectively prohibit the establishment of meaningful relationships because categories are created by the individual, not the system, and are thus fleeting and ephemeral.