How Do You Build a Thesaurus?

Building a Synonymous Search Index

The first step is to familiarize yourself with thesauri and their application in the Web environment. Fortunately, librarians and other information professionals have been building and using thesauri for quite some time, so there are many examples to review.

For an example of a very sophisticated implementation, take a look at OVID's Medline interface (select Begin Demo; you'll need to fill out a brief form to try the demo). This interface is obviously designed for expert searchers who need power and flexibility to effectively search Medline's enormous database, but it can really get you thinking about the possibilities.

For example, you can perform a normal full-text search of the database, or you can select "Map Term to Subject Heading" and run your search against the thesaurus. You can then browse the hierarchy of subject headings until you find one that matches your topic and level of specificity. In such a large database, the ability to leverage a thesaurus through this integrated searching and browsing capability really helps you to narrow your query, and find what you're looking for.

Once you're done learning from the successes and failures of others, you can begin building a thesaurus through the process of term generation and consolidation. The basic steps include:

1. Gather terms from as many sources as possible (e.g., users, subject experts, the content itself, existing thesauri). These "entry terms" should include synonyms and abbreviations, acronyms, and alternate spellings for all of the important concepts in your document collection.
2. Define the preferred terms. You'll need to create guidelines for selecting preferred terms. For example, in a collection of health-related documents that include terms such as cancer, oncology, skin, and dermatology, you'll need to decide whether to select medical terminology or regular English as the preferred terms, based on the type of language most appropriate to your primary audience. For an audience of medical professionals, you would probably select oncology and dermatology as preferred terms and cancer and skin as the respective variants. Whichever terminology you choose, it's important to be consistent in your approach to defining the preferred terms.
3. Link synonyms and near-synonyms. This is where you map the synonyms, abbreviations, acronyms, and alternate spellings as "variant terms" to the preferred terms. Within reason, the more entry terms you have, the easier it will be for indexers and users to find the preferred terms.
4. Group preferred terms by subject. This forms the foundation of your thesaurus' hierarchy. Definition of the subject hierarchy should be informed by a balance of top-down considerations (e.g., mission, vision, intended audiences) and bottom-up content analysis.
5. Identify broader and narrower terms. You're defining where each term fits within the hierarchy. Existing thesauri that cover your subject area or industry can prove extremely useful in generating ideas for broader and narrower terms.
6. Perform associative linking. The definition of related terms is highly subjective. For each term ask the question: "Where will users want to go from here?" Choose only the most obvious and important relationships.

As with everything on the Web, a thesaurus is never finished. The content and the terms used to describe concepts within that content will continue to grow and evolve. New terms must be added, old terms deleted, and relationships between terms revisited. And you should always be on the lookout for new variant terms. Thanks to Roget's Thesaurus and high school English classes, there's always someone out there with a new way to say the same thing.

If you have something to say about online thesauri, please let me know.