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Faceted classification and logical division in information retrieval
Library Trends, Wntr, 2004 by Jack Mills
An alternative method is known as retroactive notation as used in BC2. Since BC2 is used in this paper as the main vehicle for demonstrating all features of faceted classification, a brief account of retroactive notation is given here. The principle was (once again!) first used by Dewey, who reserved the zero to "introduce" three different facets (Bibliographic form, Period, and Place). So the first special subject division in a class is usually given the next digit (one) since the zero is reserved, e.g., 61 is the first subject division in class 6. In BC2 this principle of reserving earlier symbols that can then be added directly to the classmark they are qualifying is the main device for notational synthesis; it is called retroactive because synthesis in an inverted schedule is nearly always effected by qualifying one class by earlier classes--i.e., working backward (retro) in the inverted schedule, e.g., in Class S Law the following classes are found:
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* Damages (from the class Legal actions) S9M;
* Personal injury (from the class Tort) SBGQR;
* English law (from the class Common law) SN.
For the subject Damages for personal injury in English law, the classmark, built retroactively, is SNG QR9 M. Note that (1) A special provision for national jurisdictions allows all the classes in SB Substantive law to be added directly, dropping the two initial letters SB. (2) A convention to assist the easy reading of classmarks is to give the classmark in spaced multiples of three.
10.3. Hospitality to new subjects
This is, of course, an important conceptual problem (see Section 5.1), but it is often considered in notational terms. A classification system, regarded purely as a sequence of terms representing a hierarchy of conceptual classes, has no difficulty in inserting new classes once it has decided where they logically go. Just how the notation can accommodate it exactly at that theoretically desirable point is another problem. Ranganathan described it as one in which "notation brings rigidity." Remember that Ranganathan assumed an expressive (hierarchical) notation in which rigidity is certainly a major problem. In an ordinal notation, the only problem it poses is that of brevity for the new class. It does not have to bother about what the classmark looks like in terms of expressing the hierarchy.
11. THE ALPHABETICAL INDEX TO THE CLASSIFICATION
The A/Z index to the printed schedule was mentioned briefly in Section 3.2. Here, the relations between the A/Z entries, using the' natural language, and the conceptual hierarchies governing the classification itself are briefly considered. The A/Z index performs two essential functions: it provides the user of the classification with a key, linking the natural language terms for the classes to the classmarks that locate them; it complements the systematic display of relations in the hierarchy by showing under any term the distributed relatives.
The main problem is the enormous number of compound classes that are theoretically possible in a faceted classification (or even a largely enumerative one like DC), which makes it quite prohibitive to show all the distributed relatives in the A/Z index. The optimal solution to this is to recognize that the classified index and the A/Z index complement each other and that a fair division of labor is possible between the two parts. This solution is found in what Ranganathan called chain indexing (see, e.g., Mills, 1960). This has one fundamental rule--that a term in the A/Z index should never be qualified by one of its own subclasses from the classified hierarchy, e.g., using BC2, an A/Z index entry:
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