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Knowing When to Cite

Many students wonder whether they need to include a citation for every sentence they write that doesn't represent their own original thoughts.  In fact, citations are not always necessary.  Use the guidelines below to determine whether you need to cite a source in a particular situation.  When in doubt, err on the side of caution and include a citation, or check with your instructor.

What should you cite?

  • Direct quotations from sources
  • Interpretations, opinions, or ideas taken from sources
  • Statistics or results from other peoples' research studies
  • Graphics or images taken from books, articles, or websites
  • Information not commonly known to the average educated person in the field of study related to your paper topic

What should you NOT cite?

  • Your own ideas, examples, and opinions
  • Statistics or results from your own studies
  • Information that is commonly known to the average educated person in the field of study related to your paper topic ("common knowledge")

Common Knowledge

Most of these rules are fairly straightforward.  However, it can be difficult for students to recognize when a piece of information is "common knowledge," and therefore requires no citation.  When in doubt, use the following test: is the information widely available in many sources?  For example, would a general encyclopedia entry about the subject include this piece of information?  If so, it need not be cited.

Here are some examples to clarify how you might apply this rule.

  1. Basic, indisputable facts are regarded as common knowledge.  For instance:
    • Charles Darwin visited South America on the HMS Beagle, conducting surveys of the wildlife that he encountered.
    • Policies that established racially segregated schools were declared unconstitutional by the U.S. Supreme Court, as a result of its decision in Brown v. Board of Education of Topeka in 1954.
    • Earth's solar system includes an asteroid belt that lies between the orbits of Mars and Jupiter.

    Since these are factual statements, no citation is needed.

  2. Fact-based descriptions of characters or events in works of literature are regarded as common knowledge.  For instance:
    • As a child, David Copperfield was sent off to a boarding school after biting the hand of his stepfather, Mr. Murdstone.

    Since this statement describes an event that undeniably occurred in the novel David Copperfield, no citation is necessary, even if this sentence was inspired by something you read in a book or journal article.

  3. Whether a piece of information is "common knowledge" depends upon the discipline.  If the information is known by experts in the relevant field of study, it can be regarded as common knowledge when you write papers in that field.  Furthermore, an idea that is widely accepted as true by experts in the field can also be considered common knowledge, even if scholars in other disciplines do not accept it as unequivocally true.  For instance:
    • Unemployment benefits create a disincentive for job seeking.

    This idea is widely accepted in the field of economics, and therefore you would not need to include a citation for this sentence in an economics paper.  But if you make this statement in a psychology paper, you should include a citation, most likely to a textbook or reference work in economic theory.  Understanding what is common knowledge in your field of study requires experience; ask your instructor if you are unsure whether something needs to be cited.

On the other hand, if you read a source that offers opinion or interpretation regarding something that is common knowledge, and decide to incorporate this perspective into your paper, you must cite the source.  Here are some examples:

  • Darwin's observations in South America inspired his later speculations on the relationship between mind, brain, and behavior.
  • Copperfield's difficulty in extending forgiveness to his stepfather mirrored Charles Dickens's own struggles to forgive his parents for the abuse and neglect he suffered at their hands.
  • If Congress extends unemployment benefits indefinitely, the resulting disincentives for people to seek work will prevent the economy from achieving a full recovery.

All of these statements require a citation.

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