Citation Analysis: When one author cites another author, a relationship is established. Citation analysis uses citations in scholarly works to establish that relationship (links). Many different links can be ascertained, such as links between authors, between scholarly works, between journals, between fields, or even between countries. Citations both from and to a certain document may be studied. The Science Citation Index began publication in 1961.
One very common use of citation analysis is to determine the impact of a single author on a given field by counting the number of times the author has been cited by others. Citation indices, such as Institute for Scientific Information's Web of Science, allow users to search forward in time from a known article to more recent publications which cite the known item. Information scientists also use citation analysis to quantitatively assess the core journal.
Google's PageRank is based on the principle of citation analysis. Other bibliometrics applications include: creating thesauri; measuring term frequencies; exploring grammatical and syntactical structures of texts.
Data from citation indexes can be analyzed to determine the popularity and impact of specific articles, authors, and publications. However the limitation of citation analysis is that they are often incomplete or biased; data has been largely collected by hand (which is expensive), though citation indexes can also be used; incorrect citing of sources occurs continually; thus, further investigation is required to truly understand the rationale behind citing to allow it to be confidently applied.
a) Co-citation Coupling: If papers A and B are both cited by paper C, they may be said to be related to one another, even though they don't directly cite each other. If papers A and B are both cited by many other papers, they have a stronger relationship. The more papers they are cited by, the stronger their relationship is. Co-citation coupling is a method used to establish a subject similarity between two documents.
b) Bibliographic Coupling: Bibliographic coupling is the mirror image of co-citation coupling. Bibliographic coupling links two papers that cite the same articles, so that if papers A and B both cite paper C, they may be said to be related, even though they don't directly cite each other. The more papers they both cite, the stronger their relationship is.