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A citation is a reference to a source (not always the original source), published or unpublished(citation needed). A bibliographic citation is a reference to a book, article, web page, or other published item. Citations of both types should supply sufficient detail to identify the item uniquely. Different citation systems and styles are used in scientific citation, legal citation, prior art, and the humanities.

A citation number, used in some systems, is a number or symbol added inline and usually in superscript, to refer readers to a footnote or endnote that cites the source. In other citation systems, an inline parenthetical reference is used rather than a citation number, with limited information such as the author's last name, year of publication, and page number referenced; a full identification of the source will then appear in an appended bibliography.

Citation content

Citation content may include:

  • BOOK: of a book: author(s), book title, publisher, date of publication, and page number(s) if appropriate;
  • JOURNAL: of a journal article: author(s), article title, journal title, volume and issue numbers, date of publication, and page number(s);
  • NEWSPAPER: of a newspaper: author(s), article title, name of newspaper, section title and page number(s) if desired, date of publication;
  • WEB SITE: of a work on the Web: author(s), article and publication title where appropriate, as well as a URL, and a date when the site was accessed.
  • PLAY: of a play: inline citations offer part, scene, and line numbers, the latter separated by periods: 4.452 refers to scene 4, line 452. For example, "In Eugene Onegin, Onegin rejects Tanya when she is free to be his, and only decides he wants her when she is already married" (Pushkin 4.452-53).
  • POEM: of a poem: If the text is more than one line of the poem, use a slash (/) with a space before and after it to indicate the separate lines. Include the word "line" or "lines" in the Harvard reference. For example: "For I must love because I live / And life in me is what you give." (Brennan, lines 15-16).

Unique identifiers

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

Citation systems

Broadly speaking, there are two citation systems:

Parenthetical systems

In-text parenthetical citations include abbreviated source information (for example, author and page number) in parentheses in the article text. This is supplemented by complete source information in a list of Works Cited, References, or Bibliography at the end of the paper.

For example, an excerpt from the text of a paper using a parenthetical reference system might look like this:

The five stages of grief are denial, anger, bargaining, depression, and acceptance (Kubler-Ross, 1969, chap.3).

The entry in the References list would look like this:

Kubler-Ross, E. (1969). On death and dying. New York: Macmillian.

Note systems

Note systems involve the use of sequential numbers in the text which refer to either footnotes (notes at the end of the page) or endnotes (a note on a separate page at the end of the paper) which gives the 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.

For example, an excerpt from the text of a paper using a notes system without a full bibliography could look like this:

The five stages of grief are denial, anger, bargaining, depression, and acceptance.1

The note, located either at the foot of the page (footnote) or at the end of the paper (endnote) would look like this:

1. Elisabeth Kubler-Ross, On Death and Dying (New York: Macmillian, 1969), 45–60.

In a paper which contains a full bibliography, the shortened note could look like this:

1. Kubler-Ross, On Death and Dying, 45–60.

and the bibliography entry, which would be required with a shortened note, would look like this:

Kubler-Ross, Elisabeth. On Death and Dying. New York: Macmillian, 1969.

Citation styles

Citation styles can broadly be 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; Bible citation by book, chapter and verse; or Shakespeare notation by play, act and scene.

Some examples of style guides include:


  • The American Political Science Association (APSA) relies on the Style Manual for Political Science, a style often used by political science scholars and historians. It is largely based on that of the Chicago Manual of Style.
  • The ASA style of American Sociological Association is one of the main styles used in sociological publications.
  • The Chicago Style (CMOS) was developed and its guide is The Chicago Manual of Style. Some social sciences and humanities scholars use the nearly identical Turabian style. Used by writers in many fields.
  • The Columbia Style was made by Janice R. Walker and Todd Taylor to give detailed guidelines for citing internet sources. Columbia Style offers models for both the humanities and the sciences.
  • Evidence Explained: Citing History Sources from Artifacts to Cyberspace by Elizabeth Shown Mills covers primary sources not included in CMOS, such as censuses, court, land, government, business, and church records. Includes sources in electronic format. Used by genealogists and historians.
  • Harvard referencing (or author-date system) is recommended by the British Standards Institution and involves a short reference (e.g Smith, 2000) being inserted after the cited text in parenthesis and the full reference being listed at the end of the article.
  • The MHRA Style Guide is published by the Modern Humanities Research Association, and is most often used in the arts and humanities, particularly in the United Kingdom where the MHRA is based. It is fairly similar to the MLA style, but with some differences. The style guide uses footnotes that fully reference a citation and has a bibliography at the end. Its major advantage is that a reader does not need to consult the bibliography to find a reference as the footnote provides all the details. The guide is available for free download.
  • MLA style was developed by the Modern Language Association and is most often used in the humanities, particularly in English studies, comparative literature, and foreign-language literary criticism. Harvard referencing is used within the text, keyed to an alphabetical list of sources on a Works Cited page at the end of the paper. See the MLA Handbook for Writers of Research Papers.


  • The Bluebook is a citation system traditionally used in American academic legal writing, and the Bluebook (or similar systems derived from it) are used by many courts. At present, academic legal articles are always footnoted, but motions submitted to courts and court opinions traditionally use inline citations which are either separate sentences or separate clauses.


  • The ACS style is the American Chemical Society style, often used in chemistry.
  • In the AIP style of the American Institute of Physics, references are numbered in the text and the reference list.
  • The AMS styles, e.g., AMS-LaTeX, are styles developed for the American Mathematical Society (AMS), typically implemented using the BibTeX tool in the LaTeX typesetting environment. Brackets with author’s initials and year are inserted in the text and at the beginning of the reference. Typical citations are listed in-line with alphabetic-label format, e.g. [AB90]. This type of style is also called a "Authorship trigraph."
  • The Vancouver system, recommended by the Council of Science Editors, is used in medical and scientific papers and research.
    • In one major variant, citation numbers are included in the text in square brackets rather than as superscripts. All bibliographical information is exclusively included in the list of references at the end of the document, next to the respective citation number.
  • The APA style is the American Psychological Association style, which is most often used in social sciences. APA style uses Harvard referencing within the text, listing the author's name and year of publication, keyed to an alphabetical list of sources at the end of the paper on a References page.
  • Pechenik is a style described in "A Short Guide to Writing about Biology" by Jan A. Pechenik.
  • IEEE is a style used by the Institute of Electrical and Electronics Engineers which encloses citation numbers within square brackets. The reference list is arranged by the order of citation, not by alphabetical order.

Citation problems

Faulty citations include omissions of relevant papers, incorrect references, and quotation errors that misreport findings. This greatly impedes the growth of scientific knowledge because authors who fail to correctly report relevant studies are passing on false information to their readers. Furthermore, these papers are considered to be legitimate academic sources and thus more likely to be cited themselves by other papers in the future. Hence, this creates a snowball effect often leading to the proliferation of false information.

Research has shown that authors often overlook relevant research. This often occurs because they search for evidence only within their own discipline. In a study on escalation bias, papers that supported commonly-held beliefs were cited nine times more frequently than those that conflicted with common beliefs.

Research done on this subject by marketing professor J. Scott Armstrong suggests that to prevent faulty citations, authors should use the verification of citations procedure - meaning they should attempt to contact original authors to ensure that they properly cite any studies they rely on to support their main findings. Furthermore, journal editors should require authors to confirm that they have read the papers that they have cited and that they have made reasonable attempts to verify citations. This will help to reduce errors in the reference list, reduce the number of spurious references, and reduce the likelihood of overlooking relevant studies. Once a paper has been published, journals should make it easy for researchers to post relevant studies that have been overlooked. These procedures should help to ensure that new studies build properly on prior research.

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Further reading


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