Classification Scheme

In metadata a classification scheme is a hierarchical arrangement of kinds of things (classes) or groups of kinds of things. Typically it is accompanied by descriptive information of the classes or groups. A classification scheme is intended to be used for an arrangement or division of individual objects into the classes or groups. The classes or groups are based on characteristics which the objects (members) have in common. In linguistics, the subordinate concept is called a hyponym of its superordinate. Typically a hyponym is 'a kind of' its superordinate (Keith Allan, Natural language Semantics).

The ISO/IEC 11179 metadata registry standard uses classification schemes as a way to classify administered items, such as data elements, in a metadata registry.

Some quality criteria for classification schemes are:

• Whether different kinds are grouped together. In other words whether it is a grouping system or a pure classification system. In case of grouping, a subset (subgroup) does not have (inherit) all the characteristics of the superset, which makes that the knowledge and requirements about the superset are not applicable for the members of the subset.
• Whether the classes have overlaps.
• Whether subordinates (may) have multiple superordinates. Some classification schemes allow that a kind of thing has more than one superordinate others don't. Multiple supertypes for one subtype implies that the subordinate has the combined characteristics of all its superordinates. This is called multiple inheritance (of characteristics from multiple superordinates to their subordinates).
• Whether the criteria for belonging to a class or group are well defined.
• Whether the kinds of relations between the concepts are made explicit and well defined.
• Whether subtype-supertype relations are distinguished from composition relations (part-whole relations) and from object-role relations.