citation metrics

Citation impact is a measure of how many times an academic journal article or book or author is cited by other articles, books or authors. Citation counts are interpreted as measures of the impact or influence of academic work and have given rise to the field of bibliometrics or scientometrics, specializing in the study of patterns of academic impact through citation analysis. The journal impact factor, the two-year average ratio of citations to articles published, is a measure of the importance of journals. It is used by academic institutions in decisions about academic tenure, promotion and hiring, and hence also used by authors in deciding which journal to publish in. Citation-like measures are also used in other fields that do ranking, such as Google's PageRank algorithm, software metrics, college and university rankings, and business performance indicators.

Article-level

One of the most basic citation metrics is how often an article was cited in other articles, books, or other sources (such as theses). Citation rates are heavily dependent on the discipline and the number of people working in that area. For instance, many more scientists work in neuroscience than in mathematics, and neuroscientists publish more papers than mathematicians, hence neuroscience papers are much more often cited than papers in mathematics. Similarly, review papers are more often cited than regular research papers because they summarize results from many papers. This may also be the reason why papers with shorter titles get more citations, given that they are usually covering a broader area.

Most-cited papers

The most-cited paper in history is a paper by Oliver Lowry describing an assay to measure the concentration of proteins. By 2014 it had accumulated more than 305,000 citations. The 10 most cited papers all had more than 40,000 citations.
To reach the top-100 papers required 12,119 citations by 2014. Of Thomson Reuter's Web of Science database with more than 58 million items only 14,499 papers (~0.026%) had more than 1,000 citations in 2014.

Journal-level

The simplest journal-level metric is the journal impact factor (JIF), the average number of citations that articles published by a journal in the previous two years have received in the current year, as calculated by Clarivate. Other companies report similar metrics, such as the CiteScore (CS), based on Scopus.

However, very high JIF or CS are often based on a small number of very highly cited papers. For instance, most papers in ''Nature'' (impact factor 38.1, 2016) were only cited 10 or 20 times during the reference year (see figure). Journals with a lower impact (e.g. '' PLOS ONE'', impact factor 3.1) publish many papers that are cited 0 to 5 times but few highly cited articles.

Journal-level metrics are often misinterpreted as a measure for journal quality or article quality. However, the use of non-article-level metrics to determine the impact of a single article is statistically invalid. Moreover, studies of methodological quality and reliability have found that "reliability of published research works in several fields may be decreasing with increasing journal rank", contrary to widespread expectations.

Citation distribution is skewed for journals because a very small number of articles is driving the vast majority of citations; therefore, some journals have stopped publicizing their impact factor, e.g. the journals of the American Society for Microbiology.
Citation counts follow mostly a lognormal distribution, except for the long tail, which is better fit by a power law.

Other journal-level metrics include the Eigenfactor, and the SCImago Journal Rank.

Author-level

Total citations, or average citation count per article, can be reported for an individual author or researcher. Many other 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
and the g-index.
Each measure has advantages and disadvantages, spanning from bias to discipline-dependence and limitations of the citation data source. Counting the number of citations per paper is also employed to identify the authors of citation classics.

Citations are distributed highly unequally among researchers. In a study based on the Web of Science database across 118 scientific disciplines, the top 1% most-cited authors accounted for 21% of all citations. Between 2000 and 2015, the proportion of citations that went to this elite group grew from 14% to 21%. The highest concentrations of ‘citation elite’ researchers were in the Netherlands, the United Kingdom, Switzerland and Belgium. Note that 70% of the authors in the Web of Science database have fewer than 5 publications, so that the most-cited authors among the 4 million included in this study constitute a tiny fraction.

Alternatives

An alternative approach to measure a scholar's impact relies on usage data, such as number of downloads from publishers and analyzing citation performance, often at article level.

As early as 2004, the '' BMJ'' published the number of views for its articles, which was found to be somewhat correlated to citations. In 2008 the ''Journal of Medical Internet Research'' began publishing views and Tweets. These "tweetations" proved to be a good indicator of highly cited articles, leading the author to propose a "Twimpact factor", which is the number of Tweets it receives in the first seven days of publication, as well as a Twindex, which is the rank percentile of an article's Twimpact factor.

In response to growing concerns over the inappropriate use of journal impact factors in evaluating scientific outputs and scientists themselves, Université de Montréal, Imperial College London, PLOS, eLife, EMBO Journal, The Royal Society, ''Nature'' and ''Science'' proposed citation distributions metrics as alternative to impact factors.

Open Access publications

Open access (OA) publications are accessible without cost to readers, hence they would be expected to be cited more frequently. Some experimental and observational studies have found that articles published in OA journals do not receive more citations, on average, than those published in subscription journals; other studies have found that they do.

The evidence that author-self-archived ("green") OA ''articles'' are cited more than non-OA articles is somewhat stronger than the evidence that ("gold") OA ''journals'' are ci