You may have heard people say that you do not have to cite your source when the information you include is “common knowledge.” But what is common knowledge?
Broadly speaking, common knowledge refers to information that the average, educated reader would accept as reliable without having to look it up. This includes:
Information that most people know, such as that water freezes at 32 degrees Fahrenheit or that Barack Obama was the first American of mixed race to be elected president.
Information shared by a cultural or national group, such as the names of famous heroes or events in the nation’s history that are remembered and celebrated.
Knowledge shared by members of a certain field, such as the fact that the necessary condition for diffraction of radiation of wavelength from a crystalline solid is given by Bragg’s law.
However, what may be common knowledge in one culture, nation, academic discipline or peer group may not be common knowledge in another.
How do I determine if the information I am using is common knowledge?
To help you decide whether information can be considered common knowledge, ask yourself:
Who is my audience?
What can I assume they already know?
Will I be asked where I obtained my information?
A description of the symptoms of Asperger’s Syndrome would need to be cited for a composition in a general writing class but probably not need citation for an audience of graduate students in psychology.
A reference to the practice of fair value accounting would be understood by a group of economists, but would need citation to an audience of non-experts.
A statement reporting that 24% of children under the age of 18 live in households headed by single mothers would need to be cited. This is information that would not be known to the average reader, who would want to know where the figure was obtained.
The best advice is: When in doubt, cite your source.
Which of the following statements would be considered common knowledge? Which would need to be cited?
The Big Bang theory posits that the universe began billions of years ago with an enormous explosion.
The phrase “Big Bang” was coined by Sir Fred Hoyle, an English astronomer. Hoyle used the term to mock the theory, which he disagreed with.
According to the Big Bang model, the initial explosion was produced when an infinitely hot, dense center referred to as a singularity, began to expand, giving rise to the particles that eventually formed into our universe.
Statement #1 is common knowledge – the Big Bang theory is widely accepted among scientists and the term is used regularly in everyday speech.
Statement #2 needs citation; this information is very specific and may even be unknown to some physicists.
Statement #3 would not need citation to an audience of physics students but would need citation in a paper for a non-expert audience.
What is not Common Knowledge?
Datasets generated by you or others.
Statistics obtained from sources such as the US Census Bureau and the Bureau of Labor Statistics.
References to studies done by others.
Reference to specific dates, numbers, or facts the reader would not know unless s/he had done the research.
Examples of statements that need citation - each refers to work done by others, statistics, or specific information that would not be known by the average reader:
- Researchers have found that dispersants utilized to clean up oil spills can lead to lung damage when airborne particles of these dispersants combine with crude oil and are inhaled.
(Source: Wang, H., Shi, YL, Major, D. and Yang, HL (2012, August). Lung epithelial cell death induced by oil-dispersant mixtures. Toxicology in Vitro, 26, 5, 746-751. doi: 10.1016/j.tiv.2012.03.011)
- A recent study done by scholars at the Brookings Institute found that the number of people living in poverty in America grew by 12.3 million between 2000 and 2010, so that by the end of 2010, 15% of the population was living under the poverty line.
(Source: Kneebone, E., C. Nadeau and Berube, A. (2011, November 3). The re-emergence of concentrated poverty: metropolitan trends in the 2000s. Brookings Metropolitan Opportunity Series. Retrieved from http://www.brookings.edu/about/programs/metro/metropolitan-opportunity.)
- The energy of mixing per site for a binary polymer blend with differing degrees of polymerization can be described through the Flory-Huggins equation.
(Source: Flory, P.J. (1953). Principles of Polymer Chemistry. Ithaca, NY: Cornell University Press.)
Note: This equation is specific to the thermodynamics of macromolecular structures and would not be considered common knowledge by many scientists or engineers. For these reasons, they need to be cited.