Faceted classification and logical division in information retrieval

3. LEVELS OF INDEXING IN THE LIBRARY

A reader so far may have assumed that the catalog is the form par excellence of an index to the library collection and the prototype of indexes to larger collections and networks. This is not quite true. A library is indexed for retrieval at three levels: the systematic order of documents on the shelves (assuming complete or partial open access), the A/Z index to the classification governing the systematic order, and the catalog.

3.1. Shelf order

This is scarcely ever mentioned in the literature on retrieval, being treated very much as a poor relation, if not a terminally ill one. This is most unfortunate, since it is the very first index to the resources of the library for the great majority of library users and in many cases the main or even only one. Although this level of retrieval may be regarded as small beer and not deserving much attention, the special demands it makes because of its limitation to a single, linear order has had an important effect on the development of the theory of library classification. The limitation to a linear sequence throws into sharp relief a crucial property sought in indexing-that of predictability as to the location of any given class of information. The physical document can only go in one place. But the concepts that define the class represented by that one place are in most cases multiple, e.g., a class represented by the rubric Bone--Cancer--Therapy--Radiography could legitimately go in any of twenty-four different places, everyone of them making sense. The expectations of users reflect this. A radiographer would like to see it under medical radiography; the cancer specialist would like to see it under cancer, and so on. The implication is clear. The classification must have comprehensive rules governing the order in which the different component parts of a compound subject are to be taken when locating a class. This does not depend in any way on the specificity of the index descriptions given to the documents; even if the classmark locating it is not specific (i.e., reflects "broad classification") the librarian and library user still need to know where it will go--under skeletal system, or therapeutics, or radiography, or cancer.

3.2. The A/Z index to the classification

The relative index that Dewey provided for his classification has been an outstanding example of this indexing component since the scheme was first published in 1876. It intuitively recognized that the central weakness of the classified index represented by the shelf order is that it distributes many subject concepts over many fields according to the rules for combination already mentioned. So, for example, literature on children will be scattered as a result of its subordination to different containing classes-medicine, psychology, education, welfare, and so on. Hence, Dewey's (1985) term "Relative Index" and the general use of the term "distributed relatives" to describe the situation.