How to Use a Thesaurus 

The final process of indexing involves translating the relevant subject concepts of the document into the indexing language or the descriptor language of the system. The translation stage involves  the comparison of each term in the subject statement with the descriptors of the thesaurus to determine: 

1. whether an exact equivalent is available in the thesaurus, 
2. whether a broader term in the thesaurus is adequate for retrieval, 
3. whether the pre-coordinated term exists in the thesaurus (e.g. corrosion prevention, production capacity) which can be substituted for one or more terms (adjacent to each other) in a subject statement, 
4. whether it is necessary to introduce a new term  as a last resort. New terms should be introduced only after a careful consideration of terms already available in the thesaurus, the relative frequency  of occurrence of the concept in the literature, the relative frequency of its probable use in the system, its relationship to descriptors already existing in the thesauri and  its technical  precision and acceptability as determined by references to other thesauri, dictionaries, handbooks, glossaries and the like. 

After each term in a subject statement is  translated into the descriptors from the thesaurus used in a system, a set of references is established between the terms in a subject statement and the corresponding terms revealed by the thesaurus. This step is essential when the thesaurus is used as a vocabulary control tool in a system which adopts as pre-coordinated indexing system such as, PRECIS, POPSI or BTI. The two kinds of references are: 

1. See  reference (corresponding to USE reference) which directs the user to an appropriate term from, which is not used as a descriptor. A see reference is made for the non-preferred synonyms or quasi-synonymous term to the preferred lead term. 
2. See also reference (corresponding to BT to NT) to the lead term is made from its immediate broader term. A see also reference is made from the other terms which are associatively related  (M) terms, having strong mental association between that term and the lead term.