Terms in an equivalence relationship refer to the same concept (or sometimes a roughly equivalent concept). They are usually synonyms. As has already been discussed, there should be only one “indexing term” or preferred term in your thesaurus for any given concept. You can and should include synonyms of those preferred terms in your thesaurus as synonyms. And the way to do that is with equivalence relationships. The synonyms that aren’t “preferred terms” are “non-preferred terms” in the same term record. Your taxonomy software instructions will indicate how to add non-preferred terms to term records; this action will establish an equivalence relationship between the preferred term and each non-preferred term.
Equivalence relationships are done in different ways, depending on the thesaurus you are building. In a popular thesaurus, you might use Spiders as a preferred term, and Arachnids as an equivalent non-preferred term for Spiders. In a scientific thesaurus, on the other hand, you might use Arachnids as a preferred term, and Spiders as a corresponding non-preferred term. The same is true for scientific and trade names. You might use Motrin, which is a trade name, or ibuprofen, which is the scientific name. You might need to put both in a thesaurus, but the one that should be the preferred term will depend on who uses the thesaurus.
We also need to know if we will use the standard names or the slang names –sci fi versus science fiction. Normally, taxonomists use the standard names, but slang names are acceptable, especially if they are more widely used than the standard names.
In different cultures, even in what is essentially the same language, we have different terminology. Home care for the aging population is a recognized phrase in the United States, but in the United Kingdom it is domiciliary care. We have different terms, such as aerials or antennas, trunks or boots, hoods or bonnets, hire or rent.
Then we need to consider emerging concepts whose terminology has not yet stabilized, such as telecommuting or distance working (although telecommuting seems to be taking precedence). Then there are outdated terms such as washing up machines, now dishwashers.
Then we have lexical variants, which are variant spellings such as Gipsies and Gypsies. (Actually, both of those terms are now considered pejorative and so should be avoided as preferred terms; the acceptable term these days is Romani people.) Then there are Moslems or Muslims. For an effective thesaurus, you need to accommodate all such variants. The choice of which variants of ethnic and religious names to use should reflect the respective peoples’ own preferences.
If you consider the various tables for the transliteration of Arabic or any of the varieties of Chinese into English, the variations that we use depending on the transliteration tables are extraordinary. There are many different ways to spell things and, as it has been pointed out, as time goes on, the accepted spelling varies.
The Americans tend to go for an ever-more abbreviated spelling, while the British keep their long spellings. Consider the American “catalogs” and the British “catalogues”. And then there’s “weblog” (not “World Wide Web log”, mind you), coined in December 1997 by Ohioan/New Mexican Jorn Barger, and shortened to “blog” less than two years later by Californian Peter Merholz.
As for abbreviations, you need to capture both the abbreviation and the full form. Some articles might discuss ECGs or EKGs or electrocardiograms. We need to put all three forms into the same term record, with one form as the preferred term (in a medical thesaurus, the preferred term would probably be the spelled-out form) because people will use all three forms.
Then there are quasi-synonyms, which are things like urban areas versus cities, not quite the same; gifted people versus geniuses. Antonyms can also be equivalent terms: height and depth, literacy and illiteracy. If you add one, you might as well add the other one as an equivalent term, unless the granularity of your thesaurus allows you to comfortably include both terms as preferred terms.
Marjorie M.K. Hlava President, Access Innovations
This posting is one of a series based on a workshop, “Thesaurus Creation and Management,” that Marjorie Hlava presented in December of 2012.