Taxonomy (Multi-Level Vocabulary)

Definition of a Taxonomy

A multi-level controlled vocabulary in which metadata values are grouped according to subject-specific classes, usually hierarchical.

Discussion

Taxonomies are an organizational structure in which metadata values are grouped according to subject-specific classes. The structure tends to be very shallow, with increasingly narrow classes. Each class has a specific description, which is the set of characteristics that each member of a class exhibits. Taxonomies begin with the broadest of classes, and continue to narrow until the final class is reached.

The classes that are included in a taxonomy will depend, largely, upon the discipline of the metadata values. So, while a geological taxonomy might include classes like metamorphic, igeneous and sedimentary, a chemical taxonomy might include classes like solid, liquid, gas. Notice, these classes provide characteristics for each of the members of a class. The taxonomy is used to describe a particular thing, and usually culminates in a unique identifier for each member.

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

Example

Perhaps the most well-known taxonomy is the Linnaean Taxonomy, which uniquely classifies living things. At it's simplest form, the Linnaean taxonomy includes ten classes. As an example, consider the Linnaean classification for humans:

• Domain: Eukaryota
  Characteristic: organisms which have cells with a nucleus
• Kingdom: Animalia
  Characteristic: eukaryotic cells with a cell membrane but no cell wall, multi-cellular, heterotrophic
• Phylum: Chordata
  Characteristic: notochord, dorsal nerve cord, and pharyngeal gill slits
• Subphylum: Vertebrata
  Characteristic: backbone
• Class: Mammalia
  Characteristic: endothermic, hair and mammary glands (which are used to nourish the young)
• Subclass: Placentia
  Characteristic: young are born after a full gestation period
• Order: Primates
  Characteristic: collar bone, eyes facing forward, grasping hands with fingers, two types of teeth
• Family: Hominidae
  Characteristic: upright posture, large brain, stereoscopic vision, flat face, hands and feet with different functions
• Genus: Homo
  Characteristic: s-curved spine
• Species: Homo sapiens
  Characteristic: high forehead, well-developed chin, skull bones thin

To simplify the display, this sample classification for the Linnaean taxonomy is presented as a bulleted list. Notice, as human beings, we are given a unique identifier (Homo sapiens), and we exhibit all the characteristics listed (in other words, since our classification is at the bottom of a nested list, we can inherit all the characteristics of the "super-classes"). To completely classify humans in this taxonomy, we need to use the term Homo sapiens, but you could also call human beings primates. This would not be the narrowest classification, but it is an accurate classification. Taxonomies facilitate the grouping of like values, according to well-established classes.