This post discusses “common knowledge,” one of three issues that must be considered in determining whether, in a paper you’re writing, an item of information must be cited.   For information on the other two issues—types of sources and types of information that should be cited—see my upcoming posts, “Types of Information Requiring Citation” and “Types of Reference Sources Requiring Citation.”


Main Points

1.  In general, cite information sources who (or that) have informed your writing.

2.  Whether information learned from a source (e.g., an author, group of authors, or organization) should be cited depends on whether the information is “common knowledge” for your readers. Such information, if is not common knowledge for your readers, must be cited.

3.  Common knowledge is knowledge that is well established and broadly known.

4.  What is common knowledge for one group of readers may not be common knowledge for other readers. Accordingly, information that might require citation for one group of readers might not require citation for other readers.


Question: “If a ‘fact’ that I’m stating in my paper is well known, must I cite the fact?”

Answer: Maybe.

The short answer often given to this question is “When in doubt, cite.” However, this answer needs explanation. On the one hand, we should always cite sources of information who (or that) have informed our writing.  If we fail to cite our sources, readers may assess our credibility as weak and our writing as suspect. On the other hand, we should not cite information for which citation is unnecessary or inappropriate.  Unnecessary or inappropriate citation suggests that an author lacks judgment and academic maturity.

To broaden our understanding of citation, we’ll assume in this discussion that you may not be certain of (a) whether the fact is indeed undisputed and (b) the identity of the fact’s source.  Accordingly, throughout the rest of this article we’ll refer to the “fact” an assertion.

Now we can more comprehensively answer the question:  When should an assertion be cited?

Answer: Whether an assertion should be cited depends on whether the assertion is “common knowledge” for your readers. To be considered common knowledge, an assertion must be both well established and broadly known.  Both parts of this definition are important.


Well Established

Statements that appear in a scientific paper may be well founded or mere conjecture—or somewhere between these two possibilities. Thus, the degree to which an assertion is “established” lies on a continuum—from well established (i.e., strongly substantiated) to speculative. We’ve stated above that to be considered common knowledge, an assertion must be well established.

To determine whether an assertion is well established—and therefore is common knowledge not needing citation—you can consider the evidence supporting the assertion. Here you might ask: Is the assertion substantiated in the research literature? Has the assertion been substantiated by many studies? Have the studies been published in peer-review journals? Do the journals have strong credibility? What are the studies’ levels of evidence? Do the studies’ strengths and limitations regarding validity and reliability indicate that the assertion is well-substantiated? Are the research findings that substantiate the assertion long-established or newly emerging?

Main guideline: In assessing whether an item of information is substantiated well enough to be considered common knowledge, be conservative. If you are uncertain about whether a piece of information is established well enough to be considered common knowledge, provide a citation. 

Cite if the presentation of an assertion is unique.  Also, although an assertion itself may be common knowledge, the manner in which that assertion is expressed may be unique and particularly noteworthy. In such instances, the assertion’s author(s) should be cited. In addition, you might consider closely paraphrasing or directly quoting the original author’s phrasing of the assertion. 


Broadly Known

We’ve said that the second characteristic of common knowledge is that such knowledge is broadly known.  The salient question here is “Known by whom?”

The definition of common knowledge does not refer to readers in general (i.e., to the entire population of readers). Rather, the definition refers to your readers—specifically, the readers of the document in which the assertion will appear.

Note that an assertion that is common knowledge for one group of readers might not be common knowledge for another group of readers.  Thus, common knowledge for readers of nursing specialty journal will be somewhat different than common knowledge for readers of a nursing generalist journal. For example, a statement about cardiovascular care might be common knowledge for cardiovascular nurses who read the Journal of Cardiovascular Nursing—but not common knowledge for non-specialist readers of the American Journal of Nursing. Accordingly, a statement might not require citation in one journal but might require citation in another.


Ways to Determine Common Knowledge Status

One way to determine whether an assertion is common knowledge is to examine how other authors have written about the assertion. Have other recent authors cited the assertion?  If so, you probably should, too. If authors other than the assertion’s source author have not discussed (and therefore not cited) the assertion, you can examine how other authors have written about similar information.  If similar information has been cited, you should probably cite the assertion.

Note that you can examine whether the assertion is citable in two ways: either by examining relevant published content or by consulting with a subject-matter expert on the assertion.

  • Examining relevant published content. In the journal in which you’ll publish your article, is the assertion cited? Is the assertion cited in only in that one journal—or in many or all references?  If you cannot find similar information in the journal in which your assertion will appear, is similar information cited in other related journals?

If the assertion or similar information is not cited in a journal, the assertion probably is common knowledge—at least within your particular discipline of interest. If, however, similar information is cited in a journal, the assertion is probably not common knowledge—and should be cited.

Finally, do some authors cite the assertion (or similar information), but others not? Play it safe: cite the assertion.

  • Consulting with a subject-matter expert. Ask a subject-matter expert—such as the author of a textbook or reference manual on the topic—whether the assertion is common knowledge. Also, you can ask the principal investigator or “corresponding author” of a published study in which the assertion appears.  (The corresponding author is the investigator who responds to readers’ inquiries; corresponding authors’ contact information is typically provided in published studies.)


Final step: If you have determined that an assertion is not common knowledge, and you do not know the identity of the assertion’s source, you will now need to search for, identify, and cite one or more good sources.


Citation Requirement for Academic Papers

If you are a nursing student writing an academic paper (e.g., a course written assignment or a dissertation), your reading audience is usually limited: perhaps a single professor or a dissertation committee.  Such readers might—or might not—be knowledgeable about your paper’s topic or about a particular fact that is viewed by specialists as common knowledge in the specialty’s literature.

At most nursing schools, students are instructed to assume that they are writing for imaginary readers who are intelligent generalists and who do not possess specialized knowledge of the papers’ topics. In writing for this group of imaginary readers, nursing student–authors should be cautious about assuming that an assertion is common knowledge. Here the basic rule applies: When in doubt, cite.


American Psychological Association. (2010). Crediting sources. In Publication Manual of the American Psychological Association, Sixth Edition.  Washington, DC: Author.

Dartmouth University. (2018). Sources and citations at Dartmouth. Retrieved from

Massachusetts Institute of Technology. (n.d.). What is common knowledge? In Academic integrity at MIT—A handbook for students. Retrieved from

University of California–Davis. (n.d.). The meaning and prevention of plagiarism. In The Academic Integrity Project.  Retrieved from