AN EXAMINATION OF CITATION INDEXES Available to Purchase

In the past few years two methods of information access using the citations given in published papers have been developed. These methods are citation indexing and bibliographic coupling, and although they are closely related it is important clearly to distinguish between them. In essence, bibliographic coupling is a concept developed by M. M. Kessler of the Massachusetts Institute of Technology, concerned with the relation existing between two documents by virtue of their joint descent from a third—that is, two documents are said to be coupled if they both cite the same document, and the strength of the coupling is determined by the number of citations they have in common. We are here primarily interested in citation indexing and will therefore only discuss Kessler's work as it becomes immediately relevant to this subject. A citation index is ‘an ordered list of cited articles each of which is accompanied by a list of citing articles. The citing article is identified by a source citation, the cited article by a reference citation. The index is arranged by reference citations.’ It is helpful at this stage to think of the source citations as descendants, and the reference citations as their ancestors.