Hierarchical classification systems construct a tree of classifications, typically with a single root classification. Instances are then placed in the most appropriate class. A good example of this is the Dewey Decimal Classificaiton for classification of books and other information resources.

Pattern For Hierarchical Classification Systems

The pattern given here for modelling a hierarchical classification system uses one topic to represent each class in the system. The hierarchy is then modelled by creating associations between subordinate and superordinate classes. However, it is recognised that there are a wide variety of different hierarchical relationships. For this reason, the type of the associations between the subordinate and superordinate classes are not defined by this pattern. Instead, this pattern defines the type of all such types, and the type of all role types for both subordinate and superordinate role players.

The other issue is how to relate the items classified by this scheme (the instances) to the topics which represent the classes. If an instance is represented by a topic, then an association should be made between the topic representing the class and the topic representing the instance. For this purpose, topic types are introduced to represent the classification of an instance ("Classified As") and the roles played in this relationship ("Classification" and "Instance"). If the instance is not represented as a topic, then an occurrence should be used, in which case the "Instance" type can be used as an occurrence type rather than as an association role type.

The following diagram shows an example of how this pattern is employed.

Figure 11 - The Hierarchical Classification Pattern

PSIs For The Hierarchical Classification Pattern