Synonym Rings (or Search Thesaurus)

A synonym ring is a simple kind of controlled vocabulary that, as the name suggests, has controlled synonyms for concepts and nothing more. I have long included mention of synonym rings in presentations I’ve given with sections listing and describing controlled vocabulary types, and the synonym ring has appeared on diagrams illustrating comparative complexity and included features of the various controlled vocabularies, progressing from the simplest term lists to synonym rings, name authorities, taxonomies, thesauri, and finally ontologies.

However, until now, I have not gone into detail about synonym ring use and design.  

The name “synonym ring” is generally known only by taxonomists and other information professionals. It is called a “ring” because all synonyms point to each other, as in a circle or ring, rather than to a preferred term/label. Another name for it is a “search thesaurus,” although it should be clear that “thesaurus” is meant to be the Roget’s type and not the information retrieval type (similar to a taxonomy). I have also read the name “synset” but have not heard it in practice.

 

What we are talking about is a managed set of concepts, each with one or more synonyms, created specifically for supporting search, matching end-user search strings to text strings in the content being searched, for commonly searched concepts. The synonyms also match to variant names of the concept throughout the body of text that is being searched. Because the synonym ring’s purpose is to support search, it is not browsed and thus not displayed to the end users. Therefore, a preferred term or preferred label for each concept is not needed and thus not included.

Whether in a synonym ring or in another controlled vocabulary or taxonomy, “synonyms” refer to concept variants and not literal grammatical synonyms. In a controlled vocabulary, they are often phrases, not single words, and they are for things/concepts, and not all kinds of words (different parts of speech) found in a dictionary. They also don’t have to be exact synonyms, but rather sufficiently synonymous for the context of the content being searched.

Features of a synonym ring (search thesaurus)

• It includes only concepts for which there are “synonyms,” Each concept must have at least two synonyms. If there are no synonyms for the concept, then the concept is not included in the synonym ring (in contrast to a regular controlled vocabulary). So, important concepts may be absent.
• Synonyms are not displayed to the users, so slang, deprecated, potentially offensive terms, etc. may be included.
• It supports searching only and not tagging. People doing manual tagging or systems doing auto-tagging will not be able to make use of the synonyms to identify the best concept to tag with. (They could utilize another taxonomy implemented in another system for tagging.)

Implementation of synonym rings

Typically, when taxonomists are called upon to design a taxonomy, they design it with synonyms (aka alternative labels, nonpreferred terms, variants, etc.) included. Thus, creating a dedicated synonym ring type of controlled vocabulary is not common, since the necessary synonyms are already included in the taxonomy. Small taxonomies may not have synonyms, though.

Search that is built into content/record management systems may support search synonyms, but this tends to be more ad hoc than as a managed controlled vocabulary. Recently I looked into the synonym support in controlled vocabularies and taxonomies in Salesforce Service Cloud. It supports the creation of “custom synonym groups,” where each group is a synonym ring of up to six synonyms per concept, but these have to be entered individually in the user interface, rather than as an imported as a list. As such, it’s not really a “controlled vocabulary” set.

Some content management systems with included taxonomies only enable synonyms as part of their standard displayed taxonomies and not as non-displayed search synonyms. Other systems, such as SharePoint support the use of synonyms for its taxonomies (managed in its Term Store) for tagging but not for searching.

Adding search synonyms in systems that support it often have it as a systems administrator feature, which is something that the technical systems administrators may do, while taxonomists, information architects and knowledge, managers may not know about it. After all, a set of synonyms is not a “taxonomy,” so taxonomist involvement may not even be considered. Thus, communication is necessary between those who advocate the need for comprehensive search synonyms and know how best to create them and those who are in a technical role for implementing them in a system.

Advantages of synonym rings

A synonym ring is relatively easy to develop. While there are nuances to creating synonyms (described below), it’s easier than creating other controlled vocabularies or taxonomies, since there is no need to worry about which term should be preferred and how to best create a hierarchy. Since it is not displayed, getting input from users is not required.

By focusing on supporting only searching and not also tagging, the task of coming up with synonyms is also simpler, since sometimes you want synonyms to support search and not tagging and sometimes for tagging and not searching (such as when the synonyms display to users) and trying to design for both scenarios in the same taxonomy is not easy.

When searching is the primary way that users access content, rather than browsing and filtering, a synonym ring may be an ideal solution. It might not make sense to go to the effort to design and create a hierarchical taxonomy for terms that users are searching on, if the goal is to simply enhance search.

A taxonomy runs the risk of being too broad or too specific, but a synonym ring never has that issue. The size of a synonym ring type of controlled vocabulary is flexible, and it can be built out gradually over time with no detriment.

Disadvantages of synonym rings

A synonym ring is not a standard controlled vocabulary type and is not supported in the SKOS (Simple Knowledge Organization System) data model standard of the World Wide Web Consortium. This is because a SKOS controlled vocabulary (including taxonomies) needs to have preferred labels for its concepts. Thus, synonym rings are not interoperable in the same way that other controlled vocabularies are. You cannot link to external synonym rings, and you cannot even import or export them easily. They are managed within a siloed system.

Since synonym rings do not support tagging, an additional tagging controlled vocabulary with synonyms, which is somewhat redundant in its subject scope, may need to be created

Creating synonyms for a synonym ring

“Synonyms” can include dictionary synonyms, synonyms for individual words withing multi-word phrases (e.g. political protests / political demonstrations), formal and colloquial names, acronyms, etc. Following is a list of example types:

• synonyms: Cars / Automobiles
• quasi-synonyms: Learning / Training
• variant spellings: Email / E-mail
• lexical variants: Selling / Sales
• foreign language names: München / Munich
• acronyms/spelled out: GDP / Gross domestic product
• scientific/popular names: Neoplasms / Cancer
• older/current names: Near East / Middle East

Care should be taken not to include synonyms that are not sufficiently equivalent or may be vague and have other usages, such as “development” (which could refer to software development, nonprofit fundraising, or something else). It depends on context, so in the example with “tools” as a synonym software would be acceptable if the content were only about technology and not include manufacturing, construction, etc.

Synonyms can be identified when doing research for concepts to include, including manual content analysis, automatic term extraction, lists of uncontrolled keyword tags, and search log reports. Search logs are especially suitable for synonym rings, since their usage is the same: user search strings. However, often searches are on single words, whose meaning is vague. For example, a search string word of “application” is too vague and not be used as a synonym. You should only take search log search strings if their meaning is clear.

Finally, developing synonyms for a synonym ring implemented in an internal content management system is not the same as developing synonyms for a public website to support web search engine optimization (SEO), for which they are also called “search synonyms.” For SEO, web search engine algorithms need to be considered, and obtaining the greatest number of visitors is the goal, even if those site visitors did not intend to come to the website. In such cases, more specific concepts (e.g. “iPhone” as synonym for “cell phone”) as “synonyms” would be fine. If website visitors do not find what they are looking for, that’s OK. By contrast, users of enterprise CMS or search system, would consider it a waste of their time if they retrieved additional content that did not match their search. Although sample user testing is not needed, search testing to check the accuracy of results should be performed.

History of Modern Information Taxonomies

The word “taxonomy” was coined in 1813 by the Swiss botanist A. P. de Candolle, who developed a new method of classifying plants. The word is derived from the combination of Greek words τάξις (taxis), meaning “order” or “arrangement,” and νόμος (nomos), meaning “method” or “law.” The designation of taxonomy was then applied after-the-fact to Carl Linneaus’ binomial nomenclature system that had been published under the title Systema Naturae initially in 1735.

Today’s information taxonomies have their origins in a combination of classification systems, library subject heading schemes, and literature retrieval thesauri, and thus have features that combine all of these. Despite their name, information taxonomies are closer to subject heading schemes and thesauri, than they are to classification systems.

Classification systems

Classification systems have a multi-level hierarchy of classes, where a subclass is fully contained in its parent class, and consequently members of a subclass are also members of the parent class. Members (things) can belong to only one class, though. Historic examples include:

• Linnaean classification of organisms (1735-1758)
• Paris Bookseller's classification (1842)
• International Classification of Diseases (originally Bertillon Classification of Causes of Death, 1860)
• Dewey Decimal Classification (1876) and other library classifications
• Industry classification systems:
• Standard Industrial Classification System (U.S) (1937)
• International Standard Industrial Classification (U.N.) (1948)

The requirement that a thing (an organism, book, document, medical diagnosis, economic establishment) can go into only one class supports various purposes, which are not for information retrieval:

• Understanding and organism’s evolutionary background; identifying potential medicinal herbs
• Locating and reshelving a book on its shelf
• Performing heath data analysis from hospital records; billing health insurance companies appropriately
• Doing economic analysis of industries by aggregate establishment data

When it comes to information resources, classification systems may be used to determine in what (virtual) file folder a document belongs or, to support machine-learning based auto-classification.

Classification systems are also useful for data analysis, since content or records are assigned to only one classification, and this prevents any double counting. Large, data-heavy organizations might have developed their own internal classification systems for data tracking purposes. Such classifications do not serve the same purpose of a tagging/information retrieval taxonomy and should not substitute for a taxonomy but rather exist alongside for separate purposes.

Subject heading schemes

Subject heading schemes were developed to help people find books and later also articles on various subjects with more detail and flexibility for growth than classification systems. Subject headings are used for cataloguing and indexing, not for classification. Unlike classification (for shelf location) of which an item has only one classification, an item (book, article, other media) can have multiple subjects.

Features of subject heading schemes:

• Alphabetical arrangement of a very large number of subjects and/or named entities (proper nouns)
• Cross-references of See (Use) and See also (Related)
• Headings with large numbers of citations broken down to group the citations by a sub-heading or subdivision, in what is also called pre-coordination. For example, China – Foreign relations.

Back-of-the-book indexes, whose format evolved over the first half of the 20th century, follow a similar style.

Examples of early subject heading schemes:

• Library of Congress Subject Headings (1898) and other national library systems
• US. National Library of Medicine’s Medical Subject Headings (1954)

Library subject headings were adopted for periodical article indexes early on. The Reader’s Guide to Periodical Literature published by the H.W, Wilson Company had been using subject headings, including subdivisions and cross-references, since shortly after its introduction in 1901 (as can be seen in the 1900 -1905 cumulative index excerpted in the screenshot below).

(The two-digit years are from the prior century.)

Eventually, subject heading schemes adopted thesaurus features of Broader term, Narrower term, and Related term relationships, as was the case for Library of Congress Subject Headings, starting in 1985. Thus, subject heading schemes and thesauri have become very similar. The name “heading” in subject headings implies that there also exist some sub-headings/subdivisions, a feature which is not a typical of thesauri, though.

Thesauri

Information thesauri (in contrast to a dictionary thesaurus, like Roget’s) emerged in the mid-20th century outside of libraries for the more specialized subject needs of the federal government, scientific publishers, and technology companies. The word “thesaurus” was first used to refer to a controlled vocabulary, as a set of words/terms, not classification codes, for information retrieval in the 1950s.

Early thesauri include:

• E. I. Dupont de Nemours Company’s thesaurus (1959)
• Thesaurus of Armed Services Technical Information Agency (ASTIA) Descriptors, U.S. Department of Defense (1960)
• Chemical Engineering Thesaurus, published by the American Institute of Chemical Engineers (1961)

Additional professional organization publishers of scientific journals created their own thesauri in the 1960s. Dialog, the first online information service for article citations, which also utilized thesauri of information publishers, was launched in 1966.

Soon thereafter, standards for thesauri were developed and published:

• UNESCO Guidelines for the establishment and development of monolingual thesauri (1970)
• DIN 1463 (Deutsches Institut für Normung) Guidelines for the establishment and development of monolingual thesauri (1972)
• ISO 2788 Guidelines for the establishment and development of monolingual thesauri (1974) (superseded by ISO 25964-1 2011)
• ANSI American National Standard for Thesaurus Structure, Construction, and Use (1974) (superseded by ANSI/NISO Z39.19 1993)

Modern information taxonomies

The word “taxonomy” for a hierarchical structure (like a classification scheme) of terms for tagging and retrieval (like a thesaurus) gradually became popular in the 1990s. These new taxonomy-like thesauri became popular, largely due to advancements of software and website user interfaces to enable interactive displays of hierarchies. Taxonomies had the same primary purpose of thesauri, which is information findability and retrieval, but taxonomy implementations introduced new designs for browsing and expanding hierarchies. It was found that “taxonomy” also tended to resonate with business audiences better than “thesaurus.” A market for business and commercial taxonomies started to be recognized by software vendors and by consultants by the end of the 1990s.

Combining an interactive user interface with a database enabled the introduction of dynamic filters or refinements of searches by selected taxonomy terms based on different aspects, and thus faceted taxonomies emerged and have since become a popular, if not dominant, implementation of taxonomies for many different use cases. Faceted taxonomies, by combining search terms for refinement, do not need to be as large and detailed as thesauri.

As for the next chapter in the history of taxonomies, that involves a convergence with ontologies. You can read more about that in my past blog article “Taxonomies vs. Ontologies.”