One of the benefits of a taxonomy is that there are relationships between the terms to support navigating to find the most suitable term. This could be the multi-level hierarchical browsing from broader terms down to more specific narrower terms. In a strict definition of a taxonomy, the only required type of relationship between terms is hierarchical. But sometimes it may be desirable to have relationships other than hierarchical.
ISO and ANSI/NISO standards for controlled vocabularies are very explicit on the criteria for hierarchical relationships between terms. The narrower term must be a specific type of, an instance of, or an integral part of its broader term, and it must be so in all circumstances, not just sometimes. If a pair terms seem as if they should be related, but their relationship does not meet the criteria for a hierarchical relationship, then they could have an associative relationship instead, which is also known as “related term” or abbreviated as RT. Examples include Schools as related to School busses, Computers related to Operating systems, and Business development related to Marketing
Those familiar with controlled vocabularies, know that the RT relationship is a standard feature of a thesaurus, whereas it is not common in taxonomies. However, the distinction between a taxonomy and a thesaurus can be blurred, and the kind of controlled vocabulary that is designed and implemented can have features of both, serving the needs of the users and the nature of the content and indexing. Therefore, taxonomists should have a good understanding of the associative relationship, in case the need arises and the system supports it.
The creation of an RT relationship is more subjective than the creation of hierarchical relationships. Even the standard ANSI/NISO Z39.19-2005 (r2010), in section 8.4 Associative Relationships admits "The associative relationship is the most difficult to define." The standard states that the relationship is created "on the grounds that it may suggest additional terms for use in indexing or retrieval." The fact that the relationship may be created but is not required in many circumstances leaves it up to the best judgment of the taxonomist.
The only required circumstance for the RT relationship (in a thesaurus or taxonomy that has associative relationships) is between two terms that share the same broader term and they have some overlapping meaning. Examples include Boats and Ships (sharing the broader term Vessels), or Tablets and eReaders (sharing the broader term Mobile devices), or Coats and Jackets (sharing the broader term Outerwear). But there would not be a related-term relationship between Coats and Mittens, even though they are both narrower terms of Outerwear, because they don’t have overlapping meaning.
More often, however, the RT relationship is created between terms with different broader terms or even from different hierarchies entirely. The ANSI/NISO Z39.19 standard provides guidance on this only to the extent of when the relationship may be created. Whether creating the relationship is a good idea or not is up to the taxonomist’s discretion and ultimately depends on what the taxonomist thinks would be helpful to most users.
Following are examples of possible RT relationships between pairs of terms:
Baseball RT Baseball players
Ventilation RT Fans
Bacterial infections RT Antibiotics
Appliance repair RT Appliances
Plastics RT Elasticity
Timber RT Wood products
Psychology RT Psychologists
Literature RT Books
There are different users of a taxonomy/thesaurus with different goals. Depending on what they are looking for, some users would welcome the guidance of a certain RT relationship, pointing them to a related term they had not considered but find helpful (such as E-commerce RT Online shopping); whereas other users are not interested in the related term and may find extra RT relationships getting in their way. The goal is to provide RT relationships that are probably relevant and helpful to the majority of users or “the average user,” while realistically not expecting to serve all users.
The following are examples of RT relationships that are not likely to be helpful to most users and thus better not be created:
Newspapers RT Advertising
Germany RT World War II
Athletics RT Doping
A good way to become more familiar with best practices for creating RT relationships is to browse published thesauri with these relationships. Thesauri that are available publicly to browse on the web include the ERIC (Educational Resources Information Center) Thesaurus, MeSH (Medical Subject Headings), UNESCO thesaurus, and the NASA thesaurus. These and others can be found on the American Society for Indexing’s website page OnlineThesauri and Authority Files. Note that related terms might be indicated as RT, Related concepts, or See also links.
Creating associative relationships is part of the creative work of taxonomists, but taxonomists must remember that serving the user is the primary goal.