Narrower Terms vs. Alternative Terms

A number of years ago I worked on a project of cleaning up a large taxonomy on occupations and job titles. My client contact was sometimes confused between terms to be used as synonyms/variants for a preferred term and terms to be used as narrower terms to a preferred term. This initially surprised me, because the difference seemed so obvious. A more recent project raised the issue again, and I realize challenges.

The word “term” can be confusing, considering the different types of terms that exist. Both variant terms (also called synonym, nonpreferred terms, or entry terms) and narrower terms are kinds of terms. By contrast, focusing on concepts that may have various labels, the distinctions between a concept’s narrower concepts and its alternative labels is quite clear. The widely adopted SKOS (Simple Knowledge Organization System) data model standard follows the concept-based approach. SKOS is now followed by all dedicated taxonomy management software systems.

Many taxonomies, however, are not yet managed in dedicated taxonomy management systems but rather in spreadsheets or internally developed tools, neither of which follow SKOS. This is the case of both my projects in question. Each “term” in the spreadsheet-based tool had its own row, which resulted multiple rows for the same concept. Broader categories were in another column to the right. This format is potentially confusing because the variants appeared in a column as did the hierarchical levels, and you had to remember which column was which.

Regardless of the tool used, what makes it even more confusing is that a narrower concept could be either a variant term or a hierarchically narrower term. What may variously be called synonyms, variants, nonpreferred terms, entry terms, or alternative labels are not merely literal synonyms, but they could be any terms or labels that may be used in tagging to trigger the use of the concept or preferred term. This includes terms whose meaning is narrower or more specific than the term/concept in question, since the latter includes more specific terms within its scope. So, tagging the occurrence of a concept with a “broader” concept is acceptable.

For example, in a medical taxonomy a concept can be Radiation therapy. Radiotherapy is an alternative label. But then there are specific types of radiation therapy, such as Brachytherapy, Radioimmunotherapy, and Radionuclide therapy. These could be added to the taxonomy either as narrower concepts or as alternative labels to Radiation therapy, depending on how specific the taxonomy should be.

When creating or editing a taxonomy, it is often difficult to decide how specific the taxonomy should be in certain places. Terms that are too specific to warrant use as concepts should then be relegated to the status of variants/alternative labels. Deciding what is too specific depends on the concept’s relative specificity within the entire taxonomy in addition to considering the potential usage of the specific concept.

In sum, if you are not ready to adopt SKOS-based taxonomy management software, at the very least you should adopt a SKOS-based approach in conceptualizing and labeling your taxonomy. Call things “concepts” and “labels”, not “terms.” Concepts are in hierarchical relationships to each other. Labels are the names for concepts. The “preferred label” is the displayed form of the name (such as in facets in the fronted application), and “alternative labels” are variant labels to match against strings of text that may be used for the concept and trigger tagging with the concept.  Furthermore, alternative labels could be displayed differently from preferred labels, such as in italics and/or a different colored shaded cell.

 

Hierarchies in Taxonomies, Thesauri, Ontologies, and Beyond

Hierarchies are a defining feature of taxonomies, and they are also characteristic of other controlled vocabularies or knowledge organization systems, such as classification schemes, thesauri, and ontologies. The problem is that the definitions and rules for hierarchies vary depending on the kind of knowledge organization system, so you cannot assume that a hierarchy in one system converts to a hierarchy in another system.

“Hierarchy” can have various types and uses. Not all kinds of hierarchies are reflected in even in taxonomies, which tend to be quite flexible. The rules are stricter when it comes to thesauri. Finally, in ontologies, there is only one kind of hierarchy.

The hierarchies permitted in thesauri are specified in the ANSI/NISO Z39.19 and ISO 25964-1 standards, as a reciprocal inverse relationship pair of Broader term (BT) / Narrower term (NT). There are three kinds specified in these standards:

• Generic-specific  –  which refers to “is a” or “are a kind of”
      Example:
      Basketball is a kind of sport.
      Basketball BT Sports;  Sports NT Basketball
      Baketball has broader concept Sports; Sports has narrower concept Basketball
• Generic-instance – which refers to “is a named entity instance of”
     Example:
    Michael Jordan is a named basketball player.
    Jordan, Michael BT Basketball players; Basketball players NT Jordan, Michael
    Jordan, Michael has broader concept Basketball players; Basketball players has narrower concept Jordan, Michael
• Whole-part – which refers to “is in” or “is an integral part or component of”
     (not to be confused with “part” as a participant taking part in, or member of)
     Example:
     Locker rooms are in athletic facilities.
     Locker rooms BT Athletic facilities; Athletic facilities NT Locker rooms
     Locker rooms has broader concept Athletic facilities; Athletic facilities has narrower concept Locker rooms

The types of hierarchies permitted in taxonomies include all of those designated for thesauri, plus a little more flexibility due to the absence of the associative relationships. In thesauri, if the relationship between a pair of concepts is better described as associative (“Related term” - RT) than hierarchical, then they cannot be hierarchically related. In a taxonomy which lacks associative relationships, in some cases a relationship that is not accepted as hierarchical in a thesaurus may be accepted as hierarchical in a taxonomy. An example is the pair of concepts Stress and Stress management. Technically, the relationship between these two concepts is associative and not hierarchical, because Stress management is not a kind of or a part of Stress. But in a taxonomy (not a thesaurus), designating Stress management as a narrower concept of Stress may be acceptable.

As for classification schemes, despite their name, they do not always conform to class-subclass (as "is a kind of") conventions. For example, in the Dewey Decimal Classification system, 910 Geography & travel comes under 900 History. But geography and travel are not kinds of/sub-categories of history. Classification schemes may have a tendency to force a hierarchy when it’s not really an accepted taxonomic hierarchy.

Despite the looser rules for hierarchies of taxonomies and classification schemes, there are also kinds of hierarchies that are not taxonomic hierarchical. These include organizational chart hierarchies, hierarchies of (military) rank, family tree hierarchies, the ordering of social sciences concepts of as Maslow’s hierarchy of needs, or Bloom’s Taxonomy of learning objectives. The hierarchies in these cases are not broader/narrower, but rather reflect importance, influence, sequence, or some other aspect of the notion of hierarchical order. In taxonomies and thesauri, concepts in such organizational hierarchies need to be treated instead as siblings at the same level all sharing the same broader concept, such as Learning objectives as the single broader concept for all six of Bloom's learning objectives, Needs as the single broader concept for all five of Maslow's needs, Military ranks as the single broader concept for all ranks, or Job titles as the single broader concept for all job titles.

Finally, in ontologies, hierarchies may be of less significance, but they are still a feature. While relations between concepts/entities are “semantic,” with specific descriptive labels, and thus are not necessarily hierarchical, there are may be hierarchical relations between classes, when designating subclasses of classes. However, the kind of hierarchical relationship that is created between ontology classes and subclasses is limited strictly to the generic-specific type, for “is a kind of.”

Conclusions

These distinctions in hierarchies have ramifications if you want to combine, import, or convert one knowledge organization system to another. When converting a thesaurus to a taxonomy, it is possible that some of the associative relationships could be accepted as hierarchical. When converting a taxonomy to a thesaurus, existing hierarchical relationships should be reviewed to see if any should be converted to associative.

Converting a taxonomy or thesaurus to an ontology would require identifying and remove whole-part hierarchical relationships (and adding new broader concept relations to the orphaned concepts) and converting generic-instance hierarchical relationships to class-individual relationships rather than class-subclass. In fact, this may involve so much effort, which cannot be automated, that the better approach to converting a taxonomy to an ontology is probably to apply a more generic ontology as a layer to the existing taxonomy/thesaurus, which some software tools, such as PoolParty, support. Extending a taxonomy into an ontology is the subject of my next conference presentation “Ontology Design by Enriching Taxonomies” at the Data-Centric Architecture Forum on February 3.

Avoiding Mistakes in Taxonomy Hierarchical Relationships

Perhaps the most important issue in designing a hierarchical taxonomy is creating hierarchical relationships between terms correctly. This makes the taxonomy intuitively easy to understand and navigate by all kinds of users, regardless of whether they have had any training on using a taxonomy.

The basic principles of the hierarchical relationship are described in the ANSI/NISO Z39.19 and ISO 25964-1 standards for thesauri. As a quick summary, the relationship is created between terms in the following circumstances:

• a broader term which is generic and a narrower term which is a more specific type of the generic broader term,
• a broader term which is generic and the narrower term is a named instance (proper noun) of the generic broader term,
• a broader term which is a whole entity and a narrower term which is an integral part.

It is the first, generic-specific type that is most common, but is also most prone to errors by those not experienced in creating taxonomies. Typical errors include confusing refinement and narrower terms, too closely reflecting the source content hierarchy, and creating narrower terms that are applications, uses, or examples of a broader term.

Confusing Refinements with Narrower Terms

We envision users browsing a hierarchical taxonomy from top down, from broad topic to more specific topic. A more specific topic is a narrower term (NT) of a broader topic. However, instead of providing more specific topics, the creator of a taxonomy might mistakenly provide refinements of the broader topic, which are aspects of the topic, but not actually narrower terms. A term that is an aspect or refinement is not a unique stand-alone term/concept, but rather it is meant to be used in combination with its parent term.

An example of such an erroneous hierarchy would be:

  Eye diseases
  --Diagnosis

Diagnosis is an aspect or refinement of Eye diseases (and of other disease-type terms), and not a narrower term. A narrower term would be specific type of eye disease:

  Eye diseases

     NT: Glaucoma

A refinement term might not be as obvious as it is in the above example. If the same term, however, appears duplicated as a narrower term to different broader terms, but with a different implied/contextual meaning in each case, this should be red flag that the duplicated narrower term is really a refinement term. For example, the duplication of the term Waiver in a legal taxonomy as:

  Objections to evidence

  --Waiver

  Right to jury trial

  --Waiver

In this case, the duplicate narrower term should be changed to be specific in each case, such as: Objections to evidence waiver and Right to jury trial waiver.

Novice taxonomists might create such incorrect broader term-narrower term relationships because they have seen them formed as such elsewhere, such as Library of Congress Subject Headings plus Subdivisions or back-of-the-book index main entries plus subentries. A subheading or a subentry is not the same as a narrower term, because a subheading or a subentry only has usage and meaning in the context of the main heading it is associated with (appears under). A taxonomy narrower term, on the other hand, is not a different kind of term, but is rather a description of a relationship between terms. The meaning of a term in a taxonomy is constant and not dependent on its location in the taxonomy.

Too Closely Reflecting the Source Content Hierarchy

Some taxonomies are based heavily on certain text sources, such as the table of contents of one or a limited number of books or manuals, where the text is structured into units, chapters, main heading sections, subheading sections, etc. It is thus natural to make use of the structure of the text as a basis for the structure of the hierarchy. But there can be issues.

In the following example of a chapter and its headings from a textbook, greater hierarchical structure is needed for the corresponding taxonomy terms, and one of the topics (Units of Measure) does not belong within this hierarchy.

  Microbiology Laboratory

  --Microbiology Lab Personnel
  --Introduction to the Microscope

  --Units of Measure

  --Types of Microscopes

  --Laboratory Staining Methods

  --Culture Media

  --Serology

These concepts may appear in a taxonomy arranged hierarchically as follows:

  Medical laboratory technology

  NT: Laboratory equipment and supplies

       NT: Culture media

       NT: Microscopes

            NT: Microscope types

  NT: Laboratory personnel

  NT: Microscope use

       NT: Microscopy stains

  NT: Serology

  Procedures

  NT: Measurements and calculations

       NT: Units of measure

 

Another issue is that, even when the the hierarchy from the source is acceptable, the subheading-based terms are short, generic, and without context. An example is as follows:

  Eye Medications

  --Anti-infectives

  --Anti-inflammatory Agents

  --Antiglaucoma Agents

  --Local Anesthetics

The only correct narrower term above is Antiglaucoma Agents, as the other terms are not specific to eye medications. They could be linked as related terms instead.

Applications, Uses, or Example-Type Terms

Relying too much on certain text sources for the taxonomy may also result in erroneously creating narrower terms for the applications, uses, or examples of the broader term concept, because the text presents content that way.

Following are several examples:

  Web Applications

  --Tourism and Travel

  --Publishing
  --Higher Education

  --Employment

  --Financial Institutions

  --Software Distribution

  --Health Care

  Decision making issues

  --Complexity

  --Ethical conflicts

  --Information sources

  --Intraorganizational conflicts

  --Social influences

  Globalization challenges

  --Cultural differences

  --Economic risk

  --Political risk

  --Managerial limitations

Each of these so-called narrower terms are merely examples within the context of the broader term. All "narrower terms" could have other uses beyond the context of the broader term. To make the hierarchy correct, either:

1) the relationship should be changed from narrower-term (NT) to related-term (RT). This would be the case, if these terms can logically exist elsewhere in the taxonomy. Also, indexing of the concepts may require a pair of terms (such as Globalization challenges AND Economic risk),

or

2) the narrower terms should be modified and clarified, such as Cultural challenges to globalization, Economic risk challenges to globalization, Political challenges to globalization, and Managerial challenges to globalization. This would be the case, if these terms did not exist elsewhere in the taxonomy.

In conclusion, hierarchical relationships need to be constructed independent of any sources for terms, and they need to be universal and not subject to certain contexts.