Common knowledge is information that can generally be assumed to be known by the average person in a given community or discipline. However, it can be difficult to determine what information is considered common knowledge.
Your instructors and librarians recommend that you THINK CAREFULLY before deciding not to cite something. Ask yourself: Is this something I knew BEFORE doing research on my topic? Would a student in my class know the information I am claiming is common knowledge?
The Purdue Online Writing Lab (2019)offers the following guideline: "Generally speaking, you can regard something as common knowledge if you find the same information undocumented in at least five credible sources" (Deciding If Something Is Common Knowledge section).
When in doubt, it is better to cite than risk being accused of plagiarism.
Watch these two short videos to get a better understanding of what common knowledge is.
USCUpstateLibrary [screen name]. (2015, August 24). Common knowledge [video file]. Retrieved from https://www.youtube.com/watch?v=h_cAF-R4f3Y
Kirkwood Libraries [screename]. (2016, August 10). Plagiarism & citing sources: common knowledge explained [Video file]. Retrieved from https://www.youtube.com/watch?v=9if-c-8LHNk