People talk about different kinds of vocabularies. The differences usually have to do with the structure, or lack thereof. Sometimes, people refer to “flat lists”. These are one-level lists with no hierarchical structure. They can be uncontrolled or controlled lists. An uncontrolled list is a simple, flat structure. The uncontrolled list is your “Saturday list”. It is also a candidate term list; nothing particularly formatted about it. It is a simple list but it is a list. It might look something like this:

Trim bushes
Clean cat box
Water plants
Make birthday cake

It is a simple, natural language list. It can be a list of candidate terms for a taxonomy or thesaurus, serving as an excellent starting point.

Normally, an authority list is a flat list, although it can be organized by broad categories. It often defines the “approved” forms of names. The names are often of people or places. If the list is associated with variant forms of the names, it can be used to control ambiguity. It also can control ambiguity by providing a consistent form of a name for indexing.

Synonym rings try to control ambiguity, but they put forth all of the synonyms. You might have descriptor, keyword, subject headings, thesaurus term, taxonomy term, etc., all meaning roughly the same thing. You need to determine which one is the primary way you will talk about it and that is the way you will use it.

A taxonomy is solely the hierarchy, while a thesaurus tries to bring the ambiguity control, the synonyms, the hierarchies and the related terms to it. There are not many standards as yet for taxonomies (although there are many standards for thesauri).

One standard that does address taxonomy is ANSI/NISO Z39.19-2005. It defines a taxonomy as a ‘collection of controlled vocabulary terms organized into a hierarchical structure’. You may notice that it doesn’t say anything about equivalence, homograms, associated relationships, or notes. It is just the hierarchy. So as a controlled vocabulary, a parent-child or hierarchical relationship, the specificity happens at the lower levels, at the branches, at the leaflets – or at the end of the list. They are very common on websites.  They are also commonly supported as pick lists — a drop down menu of ten or twelve items. Sometimes they are browseable directories. There are many different ways to put them into play.

A thesaurus is also a controlled vocabulary. Since many thesauri are hierarchical, they may be referred to as taxonomies. However, unlike a simple taxonomy, a thesaurus does have equivalence, synonyms, associative relationships (related terms), and scope notes. It may contain definitions, editorial notes, and mappings from other thesauri and/or from taxonomies.

A thesaurus focuses on concepts. It doesn’t focus on the information object itself. You have to identify the information object, or the concept, by using the thesaurus. Rather than outlining those information objects, as a taxonomy might, it is giving you a guide to those information objects. Thus, for instance, a multilingual thesaurus term record focuses on the concept of the terms, rather than on the term itself in any one particular language. There are different ways to display a thesaurus so that you can see the network of relationships between the terms.

An ontology usually doesn’t do too well on ambiguity, but is strong on synonyms, is big on hierarchy, and it has a swarm of additional relationships — is a, as a, is part of, kinds of statements are made when you work in an ontology. Those are different kinds of relationships; restating them using Thesaurus Master software adds a different kind of related term.

Marjorie M.K. Hlava
President, Access Innovations