Page 141 - Becoming Metric Wise

132 Becoming Metric-Wise

up one level of abstraction and study relations of the form Cit(A;B,C)
and Cit(A,B;C). Cit(A;B,C) stands for A cites B and C, while Cit(A,B;C)
stands for A and B cite C.
If Cit(A,B; C) holds, i.e., articles A and B both cite article C then we
say that A and B are bibliographically coupled. The term bibliographic
coupling has been formally introduced by Kessler (1962, 1963) but the
idea dates from somewhat earlier (Fano, 1956). Indeed, using the same
underlying mathematical relation as for bibliographic coupling, Fano
pointed out that documents in a library could be grouped on the basis of
use rather than content.
Articles A and B may have other articles, besides C, occurring in their
reference list. The number of articles that their reference lists have in
common is called the bibliographic coupling strength, see Fig. 5.6. Using
the terminology of set theory we can say that the bibliographic coupling
strength of two articles is the number of elements in the intersection of
their reference lists.
The relative bibliographic coupling strength is the number of common
items divided by the number of items in the union of their two reference
list. This notion was introduced in Sen and Gan (1983). The relative bib-
liographic coupling strength is actually a Jaccard index (Jaccard, 1901).
In the language of set theory bibliographic coupling, bibliographic
coupling strength and relative bibliographic coupling strength are

Figure 5.6 Bibliographic coupling between papers X and Y and cocitation (between
e.g., papers A and B).