Categories of Controlled Vocabularies
As defined in Classification of Controlled VocabulariesA managed list of terms. In the context of vocabularies, management typically includes careful selection of terms, maintenance of terms over time (i.e. addition, deprecation, modification), and presentation of the vocabulary in an accessible format. Related Guide, vocabularies can be defined by their structure and form. There are three broad categories of controlled vocabularies: flat, multilevel, and relational (also called a relationship list). Within these three categories, there are a variety of types of controlled vocabularies. A type is a simplified name for vocabularies further classified by function. (Click on image to enlarge.)
Flat Vocabularies
Authority File * Glossary * Dictionary * Gazetteer * Code List
All flat vocabularies contain a labelA descriptor for a metadata value. This can be thought of as a question to which the value is providing an answer. For example, for the metadata label "date", the metadata value could be "March 16, 2008". and a valueMetadata values are the content connected to metadata labels in a metadata element. For example, if the metadata label is "date", the metadata value could be "May 13, 2007". Related Guide. Some flat vocabularies build upon this foundation by adding a definition or additional information about each value. No relationshipsConnections between metadata terms within a vocabulary. These relationships can connect terms by scope, provenance, or other well-defined criteria. are established, no hierarchies are set up, and no complicated matrices are necessary. Flat vocabularies are sets of two to four pieces of information: a label, a value, and possibly a definition and additional information.
Multilevel Vocabularies
A multilevel vocabulary is essentially a way to group terms into classesGrouping of metadata values, based on shared criteria. Related Guide with hierarchy. A classification tells more about the terms by placing them into well thought-out subcategories.
Think of a classification as a tree with a trunk, limbs, branches, and leaves. If you look at an individual leaf on the tree, you can backtrack to the branch, to the limb, and eventually to the trunk.
In a multilevel vocabulary, you can examine in which subcategory a term belongs, and you can examine the relationships between subcategories as well. In some multilevel vocabularies, the only connection between the subcategories is a broader than/narrower than comparison (taxonomy). In others, you can compare similar categories across broader categories (subject heading).
Relational Vocabularies
Thesaurus * Semantic Network * Ontology
Relational Vocabularies, also called relationship lists, contain a mechanism to connect terms. The relations are described by various standards and protocolsA strategy for transmitting data between systems. A protocol can be used not only over the internet, between computers, but also between applications running anywhere. Examples: FTP, SNMP, SSH. , such as for thesauriA type of relational controlled vocabulary which provides a list of terms, with specific relationships between the terms. Related Guide in the ANSIAmerican National Standards Institute /NISONational Information Standards organization Z39.19 - 2005 standard, including broader than/narrower than, used for, and related.
The principles of a relationship list can be illustrated by seashells. You might find one on the San Diego coast that looks exactly like one in Monterey Bay, except the shell in Monterey Bay has been degraded by extreme waves and the San Diego shell has not. These two are probably related (similar to broader than). Or, perhaps you see a shell on the Oregon coast, and it looks similar to a shell in Woods Hole, Massachusetts. However, the two aren't exactly the same, because they're different colors. There is still a relationship to be defined (similar to related). Perhaps you find a message in a bottle in Maryland. The bottle came from Europe, and there's a seashell inside. In this case, you would want to relate the shell to other European shells (similar to used for).