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A citation index is an index of citations between publications, allowing the user to easily establish which later documents cite which earlier documents.
The first citation indices were legal citators such as Shepard's Citations (1873). In 1960, Eugene Garfield's Institute for Scientific Information (ISI) introduced the first citation index for papers published in academic journals, starting with the Science Citation Index SCI, and later expanding to produce the Social Sciences Citation Index (SSCI) and the Arts and Humanities Citation Index (AHCI). As of 2006, there are other sources of such data, such as Google Scholar.
Major current citation indexing services
There are two publishers of general-purpose academic citation indexes, available to libraries by subscription:
- ISI is now part of Thomson Scientific. Though the ISI citation indexes are still published in print and compact disc, they are now generally accessed through the Web under the name Web of Science, which is in turn part of the group of databases in WoK.
- Elsevier publishes Scopus, available online only, which similarly combines subject searching with citation browsing and tracking in the sciences and social sciences.
There are a number of other indexes, more readily available. Some of the currently notable ones are:
- The CiteSeer system provides citation and other searching in the fields of computer and information science,
- RePec provides this in economics, and other discipline-specific indexes have also begun to include it in their indexes. Even journal publishers often supply the facility to link to late citations, at least from the journals they publish.
- Google Scholar(GS) has citation functionality, limited to the recent articles that are included. There is already discussion about the possibility that GS may in the future have sufficient capabilities to make the commercial products unnecessary, but it is not accepted that 2006 is soon enough.
Each of these products offer an index of citations between publications and a mechanism to establish which documents cite which other documents. The different products offer different ways to access the citation list and also display their citation index differently. They differ widely in cost: WOK and Scopus are among the highest-cost subscription databases; the others mentioned are free.
While citation indexes were originally designed for information retrieval purposes, they are increasingly used for bibliometrics and other studies involving research evaluation. Citation data is also the basis of the popular journal impact factor.
There is large body of literature on citation analysis, sometimes called scientometrics, a term invented by Vasily Nalimov, or more specifically bibliometrics. The field blossomed with the advent of the Science Citation Index, which now covers source literature from 1900 on. The leading journals of the field are Scientometrics and the Journal of the American Society of Information Science and Technology. ASIST also hosts an electronic mailing list called SIGMETRICS at ASIST. This method is undergoing a resurgence based on the wide dissemantion of the Web of Science and Scopus subscription databases in many universities, and the universally-available free citation tools such as CiteBase, CiteSeer, Google Scholar, and Windows Live Academic.
In a classic 1965 paper, Derek J. de Solla Price described the inherent linking characteristic of the SCI as "Networks of Scientific Papers" . The links between citing and cited papers became dynamic when the SCI began to be published online. The Social Sciences Citation Index became of the first databases to be mounted on the Dialog system  in 1972. With the advent of the CD-ROM edition, linking became even easier and enabled the use of bibliographic coupling (M. M. Kessler) for finding related records. In 1973 Henry Small published his classic work on Co-Citation analysis which became a self-organizing classification system that led to document clustering experiments and eventually an Atlas of Science later called Research Reviews.
ISI also published Current Contents, a paper publication reproducing journal title pages, widely used at the time for keeping up with the current literature, a technique known as selective dissemination of information (SDI), periodic updates of literature searches based on user profiles . The combination with SCI permitted the first use in 1965 of earlier cited references as a factor in the selection, in a product called Automatic Subject Citation Alert. This continues in electronic form as the ISI Personal Alert; this feature is now almost universally available in any bibliometric database and for most electronic journals. In the case of SCI/SSCI profiles contained not only traditional natural language search terms, but also terms for cited references and cited authors, though this too is now a part of most such systems. Thus, a user can be alerted to any new works which cited the author, paper or book in question. Using journal names in a similar way, customized contents pages could also be provided.
The inherent topological and graphical nature of the worldwide citation network which is an inherent property of the scientific literature was described by Ralph Garner at Drexel University in 1965.
The use of citation counts to rank journals was a technique used in the early part of the nineteenth century but the systematic ongoing measurement of ths counts for scientific journals was initiated by Eugene Garfield at the Institute for Scientific Information who also pioneered the use of these counts to rank authors and papers. In a landmark paper of 1965 he and Irving Sher showed the correlation between citation frequency and eminence in demonstrating that Nobel Prize winners published five times the average number of papers while their work was cited 30 to 50 times the average. In a long series of essays on the Nobel and other prizes Garfield reported this phenomenon. The usual summary measure is known as impact factor, the number of citations to a journal for the previous two years, divided by the number of articles published in those years. It is widely used, both for appropriate and inappropriate purposes--in particular, the use of this measure alone for ranking authors and papers is therefore quite controversal.
In an early study in 1964 of the use of Citation Analysis in writing the history of DNA, Garfield and Sher demonstrated the potential for generating historiographs, topological maps of the most important steps in the history of scientific topics. This work was later automated by E. Garfield, A. I. Pudovkin of the Institute of Marine Biology, Russian Academy of Sciences and V. S. Istomin of Center for Teaching, Learning, and Technology, Washington State University and led to the creation of the HistCite  software around 2002.
Autonomous citation indexing was introduced in 1998 by Giles, Lawrence and Bollacker and enabled automatic algorithmic extraction and grouping of citations for any digital academic and scientific document. Where previous citation extraction was a manual process, citation measures could now be computed for any scholarly and scientific field and document venue, not just those selected by organizations such as ISI. This led to the creation of new systems for public and automated citation indexing, the first being CiteSeer, soon followed by Cora (recently reborn as Rexa), which focused primarily on the field of computer and information science. These were later followed by large scale academic domain citation systems such as the Google Scholar and more recently Microsoft Academic. Such autonomous citation indexing is not yet perfect in citation extraction or citation clustering with an error rate estimated by some at 10%. It should be noted that SCI has also been prepared through purely programmatic methods, and the older record particularly have a similar magnitude of error.
New Measures of Citation Impact
Today, there are alternative measures of citation impact. Jorge E. Hirsch recently developed the H-index, also known as the Hirsch number, for the quantification of scientific output of individual scientific authors. It is based on the citations each article (paper) of an author gets. Hirsch writes:
- A scientist has index h if h of his/her Np papers have at least h citations each, and the other (Np - h) papers have fewer than h citations each.. (In other words, a scholar with an index of h has published h papers with at least h citations each.)
Online tools are available to directly calculate a scientist's H-index number using Google Scholar. Google Scholar and Web of Science can also be used to manually determine the H-index, although some degree of calculation is required as the H-index is the result of the maximum of the balance between the number of publications and the number of citations per publication.
The Google Pagerank system has also been proposed to assess citation impact (see Impact factor to learn more). There are an increased number of interactive tools available (such as the Scopus Citation Tracker  in 2006), and the corresponding tool for Science Citation index, Results analysis. In different ways, they rank the retrieved documents by year, journal, institution, or author.
- Impact factor
- H-index or Hirsch number
- Citation analysis
- Acknowledgment index
- Scientific journal
- The American Society for Information Science & Technology. The Information Society for the Information Age. URL accessed on 2006-05-21.
- Derek J. de Solla Price (July 30, 1965). Networks of Scientific Papers. SCIENCE 149 (3683): 510–515. PMID 14325149.
- Dialog, A Thomson Business. "Dialog invented online information services". URL accessed on 2006-05-21.
- Eugene Garfield, A. I. Pudovkin, V. S. Istomin (2002). Algorithmic Citation-Linked Historiography—Mapping the Literature of Science. Presented the ASIS&T 2002: Information, Connections and Community. 65th Annual Meeting of ASIST in Philadelphia, PA. November 18-21, 2002. URL accessed on 2006-05-21.
- An index to quantify an individual’s scientific research output. (PDF) J. E. Hirsch. URL accessed on 2006-05-21.
- Scopus Citation Tracker. Reed Elsevier Press Release: "Scopus Empowers Researchers with new Citation Tracker". URL accessed on 2006-05-21.
References & Bibliography
- Official ISI website
- Google Scholar - a scientific search engine provided by Google, makes heavy use of citation statistics.
- Google Scholar: The New Generation of Citation Indexes
- Windows Live Academic - an academic document search engine from Microsoft
- Scopus - an academic document bibliographic database from Elsevier.
- CiteSeer - autonomous citation indexing for computer and information science.
- Google Scholar: The New Generation of Citation Indexes
- Atlas of Science: Mapping Science by means of citation relations
- An Examination of Citation Counts in a New Scholarly Communication Environment
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