Wednesday, June 24, 2015

Thesaurus Principles

Thesaurus is necessary for effective information retrieval. A major purpose of a thesaurus is to match the terms brought to the system by an enquirer with the terms used by the indexer.

Whenever there are alternative names for a type of item, we have to choose one to use for indexing, and provide an entry under each of the others saying what the preferred term is. The goal of the thesaurus, and the index which is built by allocating thesaurus terms to objects, is to provide useful access points by which that record can be retrieved.

For example, if we index all full-length ladies' garments as dresses, then someone who searches for frocks must be told that they should look for dresses instead.

This is no problem if the two words are really synonyms, and even if they do differ slightly in meaning it may still be preferable to choose one and index everything under that. I do not know the difference between dresses and frocks but I am fairly sure that someone searching a modern clothing collection who was interested in the one would also want to see what had been indexed under the other. We would do this by linking the terms with the terms Use and Use for, like this:



This may be shown in a printed list, or it may be held in a computer system, which can make the substitution automatically. If an indexer assigns the term Frocks, the computer will change it to Dresses, and if someone searches for Frocks the computer will search for Dresses instead, so that the same items will be retrieved whichever term is used.

Use and Use For relationships are also used between synonyms or pairs of terms which are so nearly the same that they do not need to be distinguished in the context of a particular collection. For example:

Nuclear energy
Nuclear power

Nuclear power
Nuclear energy

Hierarchical Relationships

If we have a hundred jackets, a list under a single term will be too long to look through easily, and we should use the more specific terms. In that case, we have to make sure that a user will know what terms there are. We do this by writing a list of them under the general heading. For example:

NT (Narrower Terms)
Dinner Jackets
Flying Jackets
Sports Jackets

In the thesaurus, BT(Broader Terms)/NT relationships can be used for parts and wholes in only four special cases: parts of the body, places, disciplines and hierarchical social structures.

Good computer software should allow you to search for "Jackets and all its narrower terms" as a single operation, so that it will not be necessary to type in all the possibilities if you want to do a generic search.

Related Terms

Related terms may be of several kinds:

1. Objects and the discipline in which they are studied, such as Animals and Zoology.
2. Process and their products, such as Weaving and Cloth.
3. Tools and the processes in which they are used, such as Paint brushes and Painting.

It is also possible to use the Related Term relationship between terms which are of the same kind, not hierarchically related, but where someone looking for one ought also to consider searching under the other, e.g. Beds RT Bedding; Quilts RT Feathers; Floors RT Floor coverings.

Definitions and Scope Notes

Record information which is common to all objects to which a term might be applicable. Where there is any doubt about the meaning of a term, or the types of objects which it is to represent, attach a scope note. For example:

distinguish from Fruits as an anatomical term
includes jams
covers children up to the age of about 4 weeks; includes premature infants

Form of Thesaurus

A list based on these relationships can be arranged in various ways; alphabetical and hierarchical sequences are usually required, and thesaurus software is generally designed to give both forms of output from a single input.


a term can have several broader terms, if it belongs to several broader categories. The thesaurus is then said to be poly-hierarchical. Cardigans, for example, are simultaneously Knitwear and Jackets, and should be retrieved whenever either of these categories is being searched for.

With a poly-hierarchical thesaurus it would take more space to repeat full hierarchies under each of several broader terms in a printed version, but this can be overcome by using references, as Root does. There is no difficulty in displaying poly-hierarchies in a computerized version of a thesaurus.

Singular or Plurals

Thesaurus creation standards prescribe to use plural forms of nouns.

Use of Thesaurus

A thesaurus is an essential tool which must be at hand when indexing a collection of objects, whether by writing catalog cards by hand or by entering details directly into a computer. The general principles to be followed are:

1. Consider whether a searcher will be able to retrieve the item by a combination of the terms you allocate.
2. Use as many terms as are needed to provide required access points.
3. If you allocate a specific term, do not also allocate that term's broader terms.
4. Make sure that you include terms to express what the object is, irrespective of what it might have been used for.

If you have a computerized thesaurus, with good software, this can give you a lot of direct help. Ideally it should provide pop-up windows displaying thesaurus terms which you can choose from and then "paste" directly into the catalog record without re-typing. It should be possible to browse around the thesaurus, following its chain of relationships or displaying tree structures, without having to exit the current catalog record, and non-preferred terms should automatically be replaced by their preferred equivalents.

You should be able to "force" new terms onto the thesaurus, flagged for review later by the thesaurus editor. When editing thesaurus relationships, reciprocals should be maintained automatically, and it should not be possible to create inconsistent structures.

Thesaurus Maintenance

New terms can be suggested, and temporarily terms "forced" into the thesaurus by users. Someone has to review these terms regularly and either accept them and build them into the thesaurus structure, or else decide that they are not appropriate for use as indexing terms.

In that case they should generally be retained as non-preferred terms with USE references to the preferred terms, so that users who seek them will not be frustrated. An encouraging thought is that once the initial work of setting up the thesaurus has been done, the number of new terms to be assessed each week should decrease.

When to Use Thesaurus?

It is particularly appropriate for fields which have a hierarchical structure, such as names of objects, subjects, places, materials and disciplines, and it might also be used for styles and periods. A thesaurus would not normally be used for names of people and organisations, but a similar tool, called an authority file is usually used for these. The difference is that while an authority file has preferred and non-preferred relationships, it does not have hierarchies.

Authority files and thesauri are two examples of a generalized data structure which can allow the indication of any type of relationship between two entries, and modern computer software should allow different types of relationship to be included if needed.

Other Subject Retrieval Techniques

A thesaurus is an essential component for reliable information retrieval, but it can usefully be complemented by two other types of subject retrieval mechanism.

Classification Schemes

While a thesaurus inherently contains a classification of terms in its hierarchical relationships, it is intended for specific retrieval, and it is often useful to have another way of grouping objects. It is also often necessary to be able to classify a list of objects arranged by subject in a way which differs from the alphabetical order of thesaurus terms. Each subject group may be expressed as a compound phrase, and given a classification number or code to make sorting possible.

Free Text

It is highly desirable to be able to search for specific words or phrases which occur in object descriptions. These may identify individual items by unique words such as trade names which do not occur often enough to justify inclusion in the thesaurus. A computer system may "invert" some or all fields of the record, i.e. making all the words in them available for searching through a free-text index, or it may be possible to scan records by reading them sequentially while looking for particular words. The latter process is fairly slow, but is a useful way of refining a search once an initial group has been selected by using thesaurus terms.

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