Adjective and Verb Terms in Taxonomies

Terms in a taxonomy are generally nouns or noun phrases, but this does not mean that a taxonomy cannot comprise adjectives or verbs instead. There may be differences of opinion on this, though.

A thesaurus, another kind of controlled vocabulary, by contrast, is expected to follow standards (ANSI/NISO Z.39.19 or ISO 25964), which dictate that the terms be only nouns or noun phrases. Since a thesaurus is more structured than a taxonomy, it might be assumed that a thesaurus is a kind of taxonomy with additional features (nonpreferred terms, associative relationships, scope notes, etc.), but that the basic format of the terms are the same.  In general, this is true. Terms in the vast majority of taxonomies follow the same format as terms in thesauri, and the differences between these two different knowledge organization systems lie rather in their use of term relationships and additional attributes.

Taxonomists should attempt to follow the thesaurus standards when creating taxonomies, to the extent that is practical or relevant. Reflecting the content and serving the users are always the first priorities for taxonomies. So, there may be cases when terms as adjectives or verbs are practical.

Taxonomies vary more than thesauri do, though. While the structure of a thesaurus is consistent, taxonomies can be based on hierarchies or on facets or a combination of both. Facets are lists of terms to describe certain attributes, aspects, limit-by/filter-by categories, or metadata fields. Facets could include types such as color, size, speed, etc., in which the terms in these facets are adjectives, for example the names of individual colors.

Taxonomies with terms that are verbs are even less common than taxonomies with terms that are adjectives. Taxonomy terms of verbs (not merely verbal nouns ending in -ing) are found in only very special-purpose taxonomies. As with taxonomies with adjectives, the verb terms would not comprise or be scattered throughout an entire hierarchical taxonomy, but would rather serve as shorter term lists or facets. A good example, is Bloom’s taxonomy of educational outcomes, which is just the short list of the following verbs in this order: Remember, Understand, Apply, Analyze, Evaluate, and Create. Taxonomists might dismiss Bloom’s as not really a “taxonomy,” but it is very common to use Bloom’s terms in a facet within a faceted taxonomy for educational content.

Sets of longer verb phrases may stretch the definition of taxonomy or controlled vocabulary, but they still serve the same purpose of a controlled list within a metadata property used to tag content. This is the case for learning objectives used to tag educational content. An example of a learning objective is: “Classify costs as direct versus indirect.” Learning objectives can even be put into a hierarchy, like other taxonomies.

Metadata of phrases that begin with verbs could also be used to describe processes or procedures. I had been asked once to design a “taxonomy” for the steps and options of statements/questions to be made by sales representatives as they go through the process of achieving a sale. These “terms” would have been verbal statements similarly complex as learning objectives. The issue I had with calling it a taxonomy is that the statements would not be arranged hierarchically of broader/narrower, but rather in a flow-chart procedure format. Indeed, this would have violated the definition of a taxonomy which has to have some hierarchy. However, this would have resemblance to an ontology with its semantic relationships. So, such a procedure system still would be a kind of knowledge organization system.