Section 5
Semantic Relations

Why Indicate Semantic Relations?

Indicating semantic relations helps in several aspects of information management:
  1. checking whether a term should be used in indexing a given item or in formulating a given search specification
  2. choosing the correct level of generality in indexing and searching
  3. searching in response to broad inclusive queries
  4. sharing indexing by facilitating translation from one scheme to another

Semantic Relations between Terms

The main semantic relations indicated between preferred terms in a thesaurus are hierarchical relations and non-hierarchical relations.

BT and NT Links

BT and NT links are used to indicate hierarchical relations. In a hierarchical relation, one term is viewed as being "above" another term because it is broader in scope.

In developing a thesaurus, it is often a good idea to work out the hierarchical relations first.

When Is There a Broader/Narrower Term Relation?

There are various definitions of what constitutes a hierarchical relation. You are advised, however, to restrict yourself to the following cases.

Genus/Species

Term A is a broader term to term B (and term B is a narrower term to term A) if all the things included in the class named by term B are included in the class named by term A.

For example, "ANIMALS" is a broader term to "CATS" (and "CATS" is a narrower term to "ANIMALS") because all cats are animals.
On the other hand, "PETS" is not a broader term to "CATS" because not all cats are pets.

Class/Member

The narrower term can sometimes name a class with only one member.

For example, "UNIVERSITIES" is a broader term to "UNIVERSITY OF WESTERN ONTARIO" because The University of Western Ontario is a university.

Since thesauri usually do not include proper names, you may not encounter cases like this in constructing your own thesaurus.

Hierarchical Whole-Part

Term A is a broader term to term B (and term B is a narrower term to term A) if everything included in the class named by term B is a part of something included in the class named by term A.

For example, in a medical thesaurus, "HEAD" might be a broader term to "NOSE" because noses are normally parts of heads.

On the other hand, "FORESTS" would not be a broader term to "TREES" because not every tree is part of a forest.

Geographical Whole/Part

In a hierarchical whole/part relation, both the broader term and the narrower term may name a class with only one member. This is often true of geographical names.

For example, "NORTH AMERICA" is a broader term to "CANADA" because Canada is a part of North America.
On the other hand, "CANADA" is not a broader term to "LAKE ERIE" because only part of Lake Erie is part of Canada.

Since many thesauri do not include geographical names, you may not encounter cases like this in constructing your own thesaurus.

Summary

In recognizing hierarchical relations, you are advised to restrict yourself to the following types:
  1. genus/species
    and its special case
    class/member
  2. hierarchical whole/part
    and its special case
    geographical whole/part

quiz Quiz on hierarchical relations (requires JavaScript)


Section 4 Section 6 Table of Contents Glossary
Last updated January 25, 2008, by Tim Craven
Copyright © 1997 The University of Western Ontario