When to Cite Sources to Avoid Plagiarism

Whenever you write a paper that uses material from outside sources, you must give credit to those sources. If you take the words or information from another source and use them in a paper without proper citation, that is considered plagiarism. Plagiarism can be unintentional or on purpose, but in either case, it’s a serious offense. The word “plagiarism” comes from the Latin word for “kidnapper,” and the Modern Language Association (MLA) defines plagiarism as “the act of using another person’s ideas or expressions in your writing without acknowledging the source” (MLA Research Handbook 21). Stealing someone’s phrasing or thoughts is unethical. For this reason, universities carry strict penalties against plagiarism. Learning to cite accurately requires some practice.

Think of the information you include in your paper as falling into one of four categories: direct quotation from the author, summary or paraphrase, your opinion or argument, and common knowledge.

Below are examples of these categories and how to make a proper citation. In these examples assume you are writing an essay using the MLA style for citing sources. 

  1. A direct quotations from an author must be cited. It’s appropriate to use some direct quotations in a paper to capture phrases that you think the author has stated in a particularly effective manner. Do not go overboard and quote excessively in your paper, since your reader mainly wants to read your ideas and your words. If the reader wants to read mostly the words of your source, she can go directly to your source’s book. 

    To cite a direct quotation in MLA style, place the quotation in quotation marks, follow it with the last name of the author and the page number of the quote inside parentheses, and put a period after the closing parenthesis. 

    For example: Hitler released “the dynamic energy of the German people, reawakening their confidence in the nation and their sense of missions as a great and expanding world power” (Shirer 299).

  2. A summary or paraphrase of what the author has said must be cited. In a paraphrase you take the author’s ideas and express them entirely in your own words. A summary is similar, but it’s usually shorter than a paraphrase and highlights only the main points. For both paraphrases and summaries, MLA style requires that you include a parenthetical citation including the author’s last name and the page number(s) used at the end of the paraphrase or summary. The following examples come from the OWL at Purdue:

    The original passage:

    Students frequently overuse direct quotation in taking notes, and as a result they overuse quotations in the final [research] paper. Probably only about 10% of your final manuscript should appear as directly quoted matter. Therefore, you should strive to limit the amount of exact transcribing of source materials while taking notes.

    Lester, James D. Writing Research Papers. 2nd ed. (1976): 46-47.

    An acceptable paraphrase:

    In research papers students often quote excessively, failing to keep quoted material down to a desirable level. Since the problem usually originates during note taking, it is essential to minimize the material recorded verbatim (Lester 46-47).

    An acceptable summary:

    Students should take just a few notes in direct quotation from sources to help minimize the amount of quoted material in a research paper (Lester 46-47).

  3. Your own opinion or argument. According to “Academic Integrity at Princeton (2011),” “Often you’ll want to use facts or information to support your own argument. If the information is found exclusively in a particular source, you must clearly acknowledge that source… Note that facts are different from ideas: facts may not need to be cited, whereas ideas must always be cited. Deciding which facts or pieces of information require citation and which are common knowledge, and thus do not require citation, isn’t always easy. For example, finding the same fact or piece of information in multiple sources doesn’t necessarily mean that it counts as common knowledge. Your best course of action in such a case may be to cite the most credible or authoritative of the multiple sources.”

    For example, if you are writing an essay about the migration patterns of birds, using a phrase such as “geese tend to fly south during winter months” would be considered common knowledge and would not need a citation. However, if you use data from a scientific study conducted on the specific migration patterns of geese, then you must cite the source that you used to gain that information.

  4. Common knowledge. According to “Academic Integrity at Princeton (2011),” “You may have heard that it’s not necessary to cite a source if the information it provides is “common knowledge.” In theory, this guideline is valid… However, when you’re doing sophisticated original work at the college level, perhaps grappling with theories and concepts at the cutting edge of human knowledge, things are seldom so simple. This guideline can often lead to misunderstanding and cases of potential plagiarism. The concept of ‘common knowledge’ can never be an objective criterion for the obvious reason that what is commonly known will vary radically in different places and times… To complicate matters, each discipline has its own evolving definitions, and its own tests, for what constitutes a “fact.” And even within disciplines, experts sometimes disagree.

    The bottom line is that you may be unable to make informed decisions concerning what is and is not ‘common knowledge’… [and] especially in fields with which you’re less familiar, you must exercise caution. The belief that an idea or fact may be “common knowledge” is no reason not to cite your source.. fall back on the fundamental rule: when in doubt, cite. It’s too risky to make assumptions about what’s expected or permissible.”


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