The Purdue Online Writing Lab states that "you can regard something as common knowledge if you find the same information undocumented in at least five credible sources." Common knowledge is information that is considered widely known or easily verified. However, what is widely known can depend on your audience's culture, geographic location, age, or level of education. MIT's Handbook for Students provides several examples of information that may be considered common knowledge. When in doubt, cite your source!
Paper copies of several different style guides are available at The Desk for students to consult. Please contact us for more information.
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The 7th edition of the APA Style Manual, section 8.1 says:
"It is considered overcitation to repeat the same citation in every sentence when the source and topic have not changed. Instead, when paraphrasing a key point in more than one sentence within a paragraph, cite the source in the first sentence in which it is relevant and do not repeat the citation in subsequent sentences as long as the source remains clear and unchanged."
For more, see APA's Appropriate Level of Citation page.
The 7th edition of the APA Style Manual, section 8.25 to 8.34 says:
"It is best to paraphrase sources rather than directly quoting them because paraphrasing allows you to fit material to the context of your paper and writing style. Use direct quotes rather than paraphrasing: when reproducing an exact definition, when the author has said something memorably or succinctly, or when you want to respond to exact wording (e.g., something someone said)."
For more, see APA's Quotations page.
The 7th edition of the APA Style Manual, section 8.6 says:
"Cite secondary sources sparingly -- for instance, when the original work is out of print, unavailable, or available only in a language that you do not understand. If possible, as a matter of good scholarly practice, find the primary source, read it, and cite it directly rather than citing a secondary source. For example, rather than citing an instructor's lecture or a textbook or encyclopedia that in turn cites original research, find, read, and cite the original research directly (unless an instructor has directed you to do otherwise)."
For examples, see APA's Secondary Sources page.