A new form of document coupling called co‐citation is defined as the frequency with which two documents are cited together. The co‐citation frequency of two scientific papers can be determined by comparing lists of citing documents in the Science Citation Index and counting identical entries. Networks of co‐cited papers can be generated for specific scientific specialties, and an example is drawn from the literature of particle physics. Co‐citation patterns are found to differ significantly from bibliographic coupling patterns, but to agree generally with patterns of direct citation. Clusters of co‐cited papers provide a new way to study the specialty structure of science. They may provide a new approach to indexing and to the creation of SDI profiles.

Science mapping is discussed in the general context of information visualization. Attempts to construct maps of science using citation data are reviewed, focusing on the use of co-citation clusters. New work is reported on a dataset of about 36,000 documents using simplified methods for ordination, and nesting maps hierarchically. An overall map of the dataset shows the multidisciplinary breadth of the document sample, and submaps allow drilling down to the document level. An effort to visualize these data using advanced virtual reality software is described, and the creation of document pathways through the map is seen as a realization of Bush's (1945) associative trails. Rationale for MappingA map of science is a spatial representation of how disciplines, fields, specialties, and individual papers or authors are related to one another as shown by their physical proximity and relative locations, analogous to the way geographic maps show the relationships of political or physical features on the Earth. Lin (1997) has provided a useful typology of the various styles of representation, including hierarchical, network, scatter, and map displays. Of course there is nothing inherently two-, three-, or even N-dimensional about how scientific topics or papers relate to one another. Rather, it is a structure we impose on a collection of objects. Nevertheless, we find arranging information in space a natural and useful heuristic tool, perhaps because spatial relations play such an important role in everyday experience.Here we can only speculate on the relation of spatial cognition and information. Clearly the human mind has the ability to organize experience in spatial terms and recall objects associated with physical locations. We have all had the experience of finding an object by remembering where we placed it. Books are arranged on library shelves by topic and with related topics in close proximity. Such an arrangement facilitates browsing and finding related items, and also returning to a title we found before simply by its location. The association of a physical object with a location has a nonspatial analogue: we often recall a fact from memory by associating it with something else. It seems reasonable that creating spatial environments with information items distributed in a stable and meaningful fashion has the potential of enhancing information usability and retrieval.In the case of scientific literature, a spatial representation can facilitate our understanding of conceptual relationships and developments.

An interpretation of citation practice in scientific literature is offered which regards citation of a document as an act of symbol usage. By examining the language of the text around the footnote number the particular idea the citing author is associating with the cited document may be determined. the document is viewed as symbolic of the idea expressed in the text. This analysis was done for a sample of very highly cited documents in chemistry. A high degree of uniformity is revealed in the association of specific concepts with specific documents. These documents may be seen, in Leach's terms, as 'standard sym bols' for particular ideas, methods, and experimental data in chemical science. Some implications of these findings for the social determination of scientific knowledge (conceived as a dialogue among citing authors on the 'meaning' of earlier texts), and the relationship between cited documents as concept symbols and Kuhn's exemplars, are discussed.

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