Exhaustivity is a matter of an indexing policy and it is the measure of the extent to which all the distinct subjects are discussed in a particular document are recognized in indexing operation, and translated into the language of the system. Exhaustivity in indexing requires more number of index entries focusing different concepts (both primary and secondary) covered in the documents. The greater the number of concepts selected for indexing purpose, the more exhaustive is the indexing. If, in a given document, concepts A, B, C, D, E are selected for indexing then the indexing of the document is more exhaustive than if only concepts A< B< C are selected. When a relatively large number of concepts are indexed for each document, the policy followed is one of depth of indexing. Depth of indexing, in other words, allows for the recognition of concepts embodied not only in the main theme of the document but also in sub-themes of varying importance. Policy decision in respect of exhaustivity in indexing depends upon several factors like strength of collection, manpower available, economy and requirements of users.
In selecting a concept, the main criterion should always be its potential value as an element in expressing the subject content of the document. In making a choice of concepts, the indexer should constantly bear in mind the questions (as far as these can be known), which may be put to the information system. In effect, this criterion re-states the principal function of indexing. With this in mind, the indexer should:
• choice the concepts, which would be regarded as most, appropriate by a given community of users; and
• If necessary, modify both indexing tools and procedures as a result of feedback from enquiries.
In many cases the indexer needs to include, as part of the indexing data, concepts which are present only by implication, but which serve to set a given concept into an appropriate context.
Specificity is the degree of preciseness of the subject to express the thought content of the documents. It is the measure of the extent to which the indexing system permits the indexers to be precise when specifying the subject of the document. An indexing language is considered to be of high specificity if minute concepts are represented precisely by it. It is an intrinsic quality of the index language itself. As a rule, concepts should be identified as specifically as possible. More general concepts may be selected in some circumstances, depending upon the purpose of the information retrieval system. In particular, the level of specificity may be affected by the weight attached to a concept by the author. If the indexer considers that an idea is not fully developed, or is referred to only casually by the author, indexing at a more general level may be justified.
The quality of indexing is defined in terms of its retrieval effectiveness—the ability to retrieve what is wanted and to avoid what is not. The quality of indexing depends on two factors:
(i) The qualification of the indexer;
(ii) The quality of the indexing tools.
An indexing failure on the part of the indexer may take place at two stages of indexing process: establishing the concepts expressed in a document, and their translation. Failure in establishing concepts expressed in a document could be of two types:
a) Failure to identify a topic that is of potential interest to the target user group;
b) Misinterpretation of the content of the document, leading to the selection of inappropriate term.
The indexer should preferably be a specialist in the field for which the document is indexed. He should understand the term of the documents as well as the rules and procedures of the specific indexing system. Quality control would be achieved more effectively if the indexers have contact with users. They could then, for example, determine whether certain descriptors may produce false combinations, and also create noise at the output stage. Indexing quality is also dependent upon certain properties of the indexing method or procedure. It is essential that an index should be able to accommodate new terminology, and also new needs of users—that is, it must allow frequent updating. Indexing quality can be tested by analysis of retrieval results, e.g. by calculating recall and precision ratios.
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