Subject Heading (Multi-Level Vocabulary)

Definition of a Subject Heading

A type of multi-level controlled vocabulary in which metadata values are classified into categories which may be broad classes.

Discussion

Subject Headings are an organizational structure in which metadata values are grouped according to broad classes. This type of vocabulary provides a big picture scheme, where values can be viewed in the context of established overarching subjects. This enables a contextual grouping of values.

The classes in a subject heading are often used to provide a contextual basis for each member. Subject headings are designed for a specific purpose - a particular metadata value could be found in multiple subject headings. Each subject heading classification would provide a different context for the values. For example, you could place a fictitious metadata value "water" in multiple subject headings. It could be placed in a chemical subject heading, and include things like elements present (hydrogen and oxygen), physical state (solid, liquid, or gas), chemical properties (universal solvent, etc) - the list could go on. You could also place water in a geographic subject heading and include things like bodies of water (lake, river, ocean), extreme weather (flood, draught), water source (snow, aquifer, purification plant) - again, the list could go on. The point is that a subject heading shows how a particular value fits into an established context.

Note: While subject headings tend to include broader classes, taxonomies tend to include subject-specific, narrow classes.

Example

One of the most used subject headings is managed by the Library of Congress. The so called LCSH (Library of Congress Subject Headings) is a vocabulary that is used to catalog printed materials available in a library setting. In the taxonomy guide, the Linnaean classification for a human being was provided. The Homo sapiens label uniquely identified human beings. In the LCSH, printed media about human beings might be included in one of a variety of classes.

Library of Congress Classification Outline

Class Q - Science

Subclass QH: Natural history - Biology

QH301-705.5: Biology (General)

QH359-425: Evolution

QH426-470: Genetics

QH471-489: Reproduction

QH501-531: Life

QH540-549.5: Ecology

QH573-671: Cytology

QH705-705.5: Economic Biology

Subclass QM: Human anatomy

QM1-511: General

QM531-549: Regional anatomy

QM550-577.8: Human and comparative histology

QM601-695: Human embryology

Subclass QP: Physiology

QP1-345: General, including influence of the environment

QP351-495: Neurophysiology and neuropsychology

QP901-981: Experimental pharmacology

Subclass QR: Microbiology

QR1-74.5: General

QR180-189.5: Immunology

QR355-502: Virology

Notice, this subject heading does not uniquely identify an object - it merely classifies it according to "like objects". The classification scheme provides us with information about an object. Since the Multi-Level Vocabularies are broad, the lower objects in the lower classes do not inherit a significant amount of characateristics. For example, if something was classified with the number QH525, we would know only that it is a scientific piece about life in natural history and biology.