A citation is a reference to a source. More precisely, a citation is an abbreviated alphanumeric expression embedded in the body of an intellectual work that denotes an entry in the bibliographic references section of the work for the purpose of acknowledging the relevance of the works of others to the topic of discussion at the spot where the citation appears. Generally, the combination of both the in-body citation and the bibliographic entry constitutes what is commonly thought of as a citation (whereas bibliographic entries by themselves are not). Citations have several important purposes. While their uses for upholding intellectual honesty and bolstering claims are typically foregrounded in teaching materials and style guides (e.g., ), correct attribution of insights to previous sources is just one of these purposes. Linguistic analysis of citation-practices has indicated that they also serve critical roles in orchestrating the state of knowledge on a particular topic, identifying gaps in the existing knowledge that should be filled or describing areas where inquiries should be continued or replicated. Citation has also been identified as a critical means by which researchers establish stance: aligning themselves with or against subgroups of fellow researchers working on similar projects and staking out opportunities for creating new knowledge. Conventions of citation (e.g., placement of dates within parentheses, superscripted endnotes vs. footnotes, colons or commas for page numbers, etc.) vary by the citation-system used (e.g., Oxford, Harvard, MLA, NLM, American Sociological Association (ASA), American Psychological Association (APA), etc.). Each system is associated with different academic disciplines, and academic journals associated with these disciplines maintain the relevant citational style by recommending and adhering to the relevant style guides.

Concept

A bibliographic citation is a reference to a book, article, web page, or other published item. Citations should supply sufficient detail to identify the item uniquely. Different citation systems and styles are used in scientific citation, legal citation, prior art, the arts, and the humanities. Regarding the use of citations in the scientific literature, some scholars also put forward "the right to refuse unwanted citations" in certain situations deemed inappropriate.

Content

Citation content can vary depending on the type of source and may include:

Unique identifiers

Along with information such as authors, date of publication, title and page numbers, citations may also include unique identifiers depending on the type of work being referred to.

Systems

Broadly speaking, there are two types of citation systems, the Vancouver system and parenthetical referencing. However, the Council of Science Editors (CSE) adds a third, the citation-name system.

Vancouver system

The Vancouver system uses sequential numbers in the text, either bracketed or superscript or both. The numbers refer to either footnotes (notes at the end of the page) or endnotes (notes on a page at the end of the paper) that provide source detail. The notes system may or may not require a full bibliography, depending on whether the writer has used a full-note form or a shortened-note form. The organizational logic of the bibliography is that sources are listed in their order of appearance in-text, rather than alphabetically by author last name. For example, an excerpt from the text of a paper using a notes system without a full bibliography could look like: The note, located either at the foot of the page (footnote) or at the end of the paper (endnote) would look like this: In a paper with a full bibliography, the shortened note might look like: The bibliography entry, which is required with a shortened note, would look like this: In the humanities, many authors also use footnotes or endnotes to supply anecdotal information. In this way, what looks like a citation is actually supplementary material, or suggestions for further reading.

Parenthetical referencing

Parenthetical referencing, also known as Harvard referencing, has full or partial, in-text, citations enclosed in circular brackets and embedded in the paragraph. An example of a parenthetical reference: Depending on the choice of style, fully cited parenthetical references may require no end section. Other styles include a list of the citations, with complete bibliographical references, in an end section, sorted alphabetically by author. This section is often called "References", "Bibliography", "Works cited" or "Works consulted". In-text references for online publications may differ from conventional parenthetical referencing. A full reference can be hidden, only displayed when wanted by the reader, in the form of a tooltip. This style makes citing easier and improves the reader's experience.

Styles

Citation styles can be broadly divided into styles common to the humanities and the sciences, though there is considerable overlap. Some style guides, such as the Chicago Manual of Style, are quite flexible and cover both parenthetical and note citation systems. Others, such as MLA and APA styles, specify formats within the context of a single citation system. These may be referred to as citation formats as well as citation styles. The various guides thus specify order of appearance, for example, of publication date, title, and page numbers following the author name, in addition to conventions of punctuation, use of italics, emphasis, parenthesis, quotation marks, etc., particular to their style. A number of organizations have created styles to fit their needs; consequently, a number of different guides exist. Individual publishers often have their own in-house variations as well, and some works are so long-established as to have their own citation methods too: Stephanus pagination for Plato; Bekker numbers for Aristotle; citing the Bible by book, chapter and verse; or Shakespeare notation by play. The Citation Style Language (CSL) is an open XML-based language to describe the formatting of citations and bibliographies.

Humanities

In some areas of the humanities, footnotes are used exclusively for references, and their use for conventional footnotes (explanations or examples) is avoided. In these areas, the term footnote is actually used as a synonym for reference, and care must be taken by editors and typesetters to ensure that they understand how the term is being used by their authors.

Law

Sciences, mathematics, engineering, physiology, and medicine

Social sciences

Issues

In their research on footnotes in scholarly journals in the field of communication, Michael Bugeja and Daniela V. Dimitrova have found that citations to online sources have a rate of decay (as cited pages are taken down), which they call a "half-life", that renders footnotes in those journals less useful for scholarship over time. Other experts have found that published replications do not have as many citations as original publications. Another important issue is citation errors, which often occur due to carelessness on either the researcher or journal editor's part in the publication procedure. For example, a study that analyzed 1,200 randomly selected citations from three major business ethics journals concluded that an average article contains at least three plagiarized citations when authors copy and paste a citation entry from another publication without consulting the original source. Experts have found that simple precautions, such as consulting the author of a cited source about proper citations, reduce the likelihood of citation errors and thus increase the quality of research. Another study noted that approximately 25% citations do not support the claims made, a finding that affects many disciplines, including history. Research suggests the impact of an article can be, partly, explained by superficial factors and not only by the scientific merits of an article. Field-dependent factors are usually listed as an issue to be tackled not only when comparisons across disciplines are made, but also when different fields of research of one discipline are being compared. For example, in medicine, among other factors, the number of authors, the number of references, the article length, and the presence of a colon in the title influence the impact; while in sociology the number of references, the article length, and title length are among the factors. Studies of methodological quality and reliability have found that "reliability of published research works in several fields may be decreasing with increasing journal rank". Nature Index recognizes that citations remain a controversial and yet important metric for academics. They report five ways to increase citation counts: (1) watch the title length and punctuation; (2) release the results early as preprints; (3) avoid referring to a country in the title, abstract, or keywords; (4) link the article to supporting data in a repository; and (5) avoid hyphens in the titles of research articles. Citation patterns are also known to be affected by unethical behavior of both the authors and journal staff. Such behavior is called impact factor boosting and was reported to involve even the top-tier journals. Specifically the high-ranking journals of medical science, including The Lancet, JAMA and The New England Journal of Medicine, are thought to be associated with such behavior, with up to 30% of citations to these journals being generated by commissioned opinion articles. On the other hand, the phenomenon of citation cartels is rising. Citation cartels are defined as groups of authors that cite each other disproportionately more than they do other groups of authors who work on the same subject.

Research and development

There is research about citations and development of related tools and systems, mainly relating to scientific citations. Citation analysis is a method widely used in metascience.

Citation analysis

Citation frequency

Progress and citation consolidation

IT systems