Common knowledge, or "what everybody knows", is the only thing that does not need to be cited.
How does one know, however, "what everybody knows"?
Generally, for a fact to be considered common knowledge, it has to meet two criteria:
- It must have been published in at least three independent sources; and
- It must be known by a wide variety of people.
Ideally, it should meet a third criteria--that is, the fact may be found in general reference sources--general encyclopedias or almanacs. Bear in mind, too, that a fact that is considered "common knowledge" to someone within a particular field will not be considered common knowledge to someone outside of the field.
Controversial?
With controversial issues, common knowledge is factual and must involve agreement among most people. "It is NOT common knowledge that drilling will affect caribou migration or feeding habits." While evidence may exist to support this statement, there is not enough agreement to make it "common knowledge."
"Common Knowledge". Cornell University.
