Willpower Information Information Management Consultants

Thesaurus principles and practice

Contents

1. Why do we need a thesaurus?
   
2. A limited list of indexing terms
   
3. Hierarchical relationships
   
4. Related terms
   
5. Definitions and scope notes
   
6. Form of the thesaurus
   
7. Special factors relating to museum objects
   
   • Abstract terms and disciplines
     
   • Singular or plural terms
     
   • Parts and wholes
     
   • Polyhierarchies
     
8. Use of a thesaurus when cataloguing
   
9. Use and modification of existing thesauri
10. Thesaurus maintenance
11. What sort of fields is a thesaurus appropriate for?
12. Other subject retrieval techniques
    • Classification schemes
    • Free text

• Bibliography on thesaurus construction and use

1 Why do we need a thesaurus?

The simple answer is that we give each item a "name", and then we can create a file of index cards, or a computer file, in which we can search for these names and expect to find all the appropriate items. This is the concept of the simple name field in the MDA data structure. It is straightforward at first, and seems intuitive, but once you have documentation which has been built up over time, perhaps by many different people, problems creep in unless there are rules and guidelines to maintain consistency.

The word thesaurus is a rather fancy name, which has acquired a certain mystique, because it is often bandied about as something necessary for effective information retrieval, but something which sounds as though it will involve a lot of work. I have often heard curators say "That's all very well if you have the time and resources, but I have this great backlog of cataloguing to do, and I would never get through the half of it if I had to spend time setting up anything as complicated as a thesaurus. What I need is a simple list of names which I can use to index my objects."

My main purpose in this paper is to make three points:

• A simple name list without some rules will rapidly become a mess.
• Only three simple rules are needed; using them will make life easier for you, not harder.
• So long as you stick to these rules, you can take an existing thesaurus and adapt it to your needs; you are not limited to using the terms which are listed in it already, and you are not obliged to use more detail than you need.

1. Use a limited list of indexing terms, but plenty of entry terms
   -- link these with USE and USE FOR (UF) relationships.
2. Structure terms of the same type into hierarchies
   -- link these with BROADER TERM/NARROWER TERM (BT/NT) relationships.
3. Remind users of other terms to consider
   -- link these with RELATED TERM/RELATED TERM (RT/RT) relationships.

Contents list

Home page

2 A limited list of indexing terms

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 normally do this by linking the terms with the terms USE and USE FOR , thus:

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. A friendly computer will explain what it is doing, so that the user is not puzzled by being given items with terms different from those asked for.

USE and USE FOR relationships are thus 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. Other examples might be:

If we name objects, we want to be as specific as possible. If we have worked hard to discern subtle distinctions in nature, type or style, we certainly want to record these. The point is that the thesaurus is not the place to do this. Detailed description of an object is the job of the catalogue record; the job 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.

USE and USE FOR relationships can also be used to group similar items together, because too much specificity is as bad as too little. If we have a small clothing collection, containing ten jackets, it is more useful to give them all the index term jackets than to create many specific categories. Anyone searching our catalogue will then be able to search on the single term jackets and see a list of the ten items, each with a description of exactly what kind of jacket it is, as follows:

If we used all the possible specific names, each of which would have only one or two items in it, such as blazers, dinner jackets, boleros, donkey jackets, anoraks, flying jackets, sports jackets, and so on, enquirers would have to search the catalogue under each name in turn in order to find all the jackets in the collection, and they would never be sure that there was not a kind of jacket that they had overlooked.

To help enquirers who approach the system by one of these terms, we therefore create the references:

Contents list

Home page

3 Hierarchical relationships

We could just invert terms and rely on the alphabet to bring them together, in a list such as

but this is unreliable and subject to the vagaries of the language, which does not always describe a specific type of item by an adjective preceding the generic name. We have to accommodate types of jacket which have their own distinctive names such as Anoraks or Blazers.

In both the above cases, it is important that the terms which are linked are of the same type. That is to say that any narrower term must be a specific case of the broader term, and able to inherit its characteristics. (The developers of Object Oriented Programming have recently discovered this idea, which has been known to the worlds of information science and biological taxonomy for a very long time.) Thus if we say that Blazers is a narrower term of Jackets, we mean that every blazer is, whatever else it may be, inherently a jacket, and that it has the characteristics which define a jacket. Mice can properly be said to be a narrower term of Rodents, because all mice are inherently rodents, but it is not correct to list Mice as a narrower term of Pests, because some mice, such as laboratory mice and pet mice, are not pests. The idea is to have relationships in the thesaurus which are always true, irrespective of context. In the same way, it would not be correct to list Buses as a narrower term of Diesel-engined vehicles, although many of them are; if we have a diesel-engined bus in our collection, we should show this by giving it the two terms Buses and Diesel-engined vehicles.

Broader and narrower terms Hierarchical relationships Relationships must be independent of context Terms must represent the same type of entity Mice BT Rodents Rodents NT Mice Shoes BT Footwear Footwear NT Shoes Mice BT Pests Pests NT Mice Shoes BT Shoemaking Shoemaking NT Shoes

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:

Contents list

Home page

4 Related terms

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.

Contents list

Home page

5 Definitions and scope notes

A thesaurus is not a dictionary, and it does not normally contain authoritative definitions of the terms which it lists. It could perfectly well do this, but a lot more work would be required to develop it in this way. In an automated system, however, the thesaurus would be a logical place to record information which is common to all objects to which a term might be applied, for example notes on the history and origin of Anoraks or the identifying characteristics and lifestyle of Mice (or perhaps Mus musculus in a taxonomic thesaurus). Where there is any doubt about the meaning of a term, or the types of objects which it is to represent, a SCOPE NOTE (SN) is attached to it. For example, Fruit SN distinguish from Fruits as an anatomical term BT Foods Preserves SN includes jams Neonates SN covers children up to the age of about 4 weeks; includes premature infants 6 Form of the 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 typical simple thesaurus of a few clothing terms is shown in Tables 1 and 2.

Table 1: Sample thesaurus - hierarchical sequence knitwear > cardigans > pullovers outerwear > blouses > cardigans > coats > > raincoats > dresses > jackets > > anoraks > > blazers > > dinner jackets > > donkey jackets > > reefer jackets > leggings > pullovers > rainwear > > raincoats > shawls > shirts > skirts > suits > trousers > > jeans > > shorts > > slacks

Table 2: Sample thesaurus - alphabetical sequence
anoraks BT jackets blazers BT jackets blouses UF BT smocks outerwear breeches USE trousers capes USE coats cardigans SN knitted jackets with front opening BT knitwear outerwear cloaks USE coats coats UF capes cloaks overcoats BT outerwear NT raincoats dinner jackets BT jackets	donkey jackets BT jackets dresses UF BT frocks outerwear duffel jackets USE reefer jackets frocks USE dresses jackets BT outerwear NT anoraks blazers dinner jackets donkey jackets reefer jackets jeans BT trousers jumpers USE pullovers knitwear NT cardigans pullovers leggings BT outerwear	outerwear NT blouses cardigans coats dresses jackets leggings pullovers rainwear shawls shirts skirts suits trousers overcoats USE coats pullovers UF jumpers sweaters BT knitwear outerwear raincoats BT coats rainwear rainwear BT outerwear NT raincoats reefer jackets UF duffel jackets BT jackets	shawls UF wraps (clothing) BT outerwear shirts BT outerwear shorts BT trousers skirts BT outerwear slacks BT trousers smocks USE blouses suits BT outerwear sweaters USE pullovers trousers UF breeches BT outerwear NT jeans shorts slacks wraps (clothing) USE shawls

Contents list

Home page

7 Special factors relating to museum objects

7.1 Abstract terms and disciplines

You may well wish to allocate abstract and discipline terms to objects too, so that you can retrieve all the objects to do with Dentistry, Laundry, Warfare or Food preparation. These terms can also be included in the thesaurus, so long as they are not given hierarchical relationships to names of objects. They should be given RT relationships to an appropriate level of object terms.

Some thesauri, such as ROOT, interfile terms of different types in their hierarchical display. Indentation in such cases does not necessarily indicate a BT/NT relationship. The relationships are shown in ROOT's alphabetical sequence, and it is unfortunate that they are not distinguished in the hierarchical one.

Because these abstract terms do not describe what the object is, they could be put into a field in the catalogue record labelled concept or subject, distinct from the field containing terms which name the object. I do not think that such a distinction will generally be helpful to users, however, and there seems to be no disadvantage in putting both types of term into a single field so that they can easily be searched as alternatives or in combination. Such a field would not be correctly called name and I therefore prefer to call it simply indexing terms or subject indexing terms.

7.2 Singular or plural terms

The point is not a trivial one, because as discussed in section 2 above there is a conceptual difference between naming or describing an object and grouping it with others so that it can be found. Both are essential steps, but an information retrieval thesaurus is primarily concerned with grouping.

Singular or plural terms?
The cataloguer thinks: "This is a clock".
The enquirer asks: "What clocks do you have?"
Prefer plural terms because: We should design the catalogue to fit the way the user thinks. Clocks is the name of a category, including many types, so plural is more logical.

The British Standard for thesaurus construction recommends that plural terms should be used, except for a few well-defined cases, and my view is that this practice should be followed. Unfortunately, there are many records in museum collections which have been given singular "object names", and the work of changing these to plurals in a move to a thesaurus structure may be so great as to require some compromise.

7.3 Parts and wholes

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

7.4 Polyhierarchies

Art and architecture thesaurus (AAT), regrettably, allows a term to have only one broader term, and I think that this is a serious drawback to its usefulness as an information retrieval tool. 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 polyhierarchies in a computerised version of a thesaurus.

Contents list

Home page

8 Use of a thesaurus when cataloguing

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.

9 Use and modification of existing thesauri

Even when using an authoritative thesaurus, some care is needed, and I have mentioned some limitations of ROOT and AAT in 7.1 and 7.4 above. It is still much easier to base your work on something like these than to build your own from scratch, unless you have a very specialised collection.

10 Thesaurus maintenance

Contents list

Home page

11 What sort of fields is a thesaurus appropriate for?

[Authority files and thesauri are two examples of a generalised 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.]

12 Other subject retrieval techniques

12.1 Classification schemes.

12.2 Free text.

This document is at http://www.willpowerinfo.co.uk/thesprin.htm
Revised 13th February 1998

Comments and feedback on content or presentation are welcome and should be sent to Leonard Will at L.Will@willpowerinfo.co.uk

Copyright © Willpower Information, 1998.

Contents list

Home page