Citation impact
Citation is the process of acknowledging or citing the author, year, title, and locus of publication (journal, book, or other) of a source used in a published work. Such citations can be counted as measures of the usage
Usage is the manner in which written and spoken language is used. H. W. Fowler's Dictionary of Modern English Usage "defines usage as 'points of grammar, syntax, style, and the choice of words'". The Oxford Dictionary of English defines usage as "the way in which a word or phrase is normally and...

 and impact of the cited work. This is called citation analysis
Citation analysis
Citation analysis is the examination of the frequency, patterns, and graphs of citations in articles and books. It uses citations in scholarly works to establish links to other works or other researchers. Citation analysis is one of the most widely used methods of bibliometrics...

 or bibliometrics
Bibliometrics is a set of methods to quantitatively analyze scientific and technological literature. Citation analysis and content analysis are commonly used bibliometric methods...

. Among the measures that have emerged from citation analysis are the citation counts for:
  • an individual article (how often it was cited);
  • an author (total citations, or average citation count per article);
  • a journal
    Academic journal
    An academic journal is a peer-reviewed periodical in which scholarship relating to a particular academic discipline is published. Academic journals serve as forums for the introduction and presentation for scrutiny of new research, and the critique of existing research...

     (average citation count for the articles in the journal).

Many measures have been proposed, beyond simple citation counts, to better quantify an individual scholar's citation impact. The best-known measures include the h-index
The h-index is an index that attempts to measure both the productivity and impact of the published work of a scientist or scholar. The index is based on the set of the scientist's most cited papers and the number of citations that they have received in other publications...

 and the g-index
The g-index is an index for quantifying scientific productivity based on publication record. It was suggested in 2006 by Leo Egghe.The index is calculated based on the distribution of citations received by a given researcher's publications:...

. Each measure has advantages and disadvantages, spanning from bias to discipline-dependence and limitations of the citation data source.

An alternative approach to measure a scholar's impact relies on usage data, such as number of downloads from publishers.

An important recent development in research on citation impact is the discovery of universality, or citation impact patterns that hold across different disciplines in the sciences, social sciences, and humanities. For example it has been shown that the number of citations received by a publication, once properly rescaled by its average across articles published in the same discipline and in the same year, follows a universal log-normal distribution that is the same in every discipline. This finding has suggested a universal citation impact measure that extends the h-index by properly rescaling citation counts and resorting publications, however the computation of such a universal measure requires the collection of extensive citation data and statistics for every discipline and year. Social crowdsourcing
Crowdsourcing is the act of sourcing tasks traditionally performed by specific individuals to a group of people or community through an open call....

 tools such as Scholarometer have been proposed to address this need.

While citation counts are often correlated with other measures of scholarly and scientific performance, causal statements linking a citation advantage with open access status have been contradicted by some experimental and observational studies.

Automated citation index
Citation index
A citation index is a kind of bibliographic database, 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...

ing has changed the nature of citation analysis research, allowing millions of citations to be analyzed for large scale patterns and knowledge discovery. The first example of automated citation indexing was CiteSeer
CiteSeer was a public search engine and digital library for scientific and academic papers. It is often considered to be the first automated citation indexing system and was considered a predecessor of academic search tools such as Google Scholar and Microsoft Academic Search. It was replaced by...

, later to be followed by Google Scholar
Google Scholar
Google Scholar is a freely accessible web search engine that indexes the full text of scholarly literature across an array of publishing formats and disciplines. Released in beta in November 2004, the Google Scholar index includes most peer-reviewed online journals of Europe and America's largest...

. More recently, advanced models for a dynamic analysis of citation aging have been proposed. The latter model is even used as a predictive tool for determining the citations that might be obtained at any time of the lifetime of a corpus of publications.

Other readings

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