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When to Cite a Source

Knowing When to Support Your Statements


"Write an essay and back it up with facts."

How many times have you heard a teacher or professor say this? But many students might wonder what exactly counts as a fact, and what doesn't. That means they don't know when it is proper to cite a source, and when it's OK not to use a citation.

Dictionary.com states that a fact is:


  • Something demonstrated to exist or known to have existed.


"Demonstrated" is a hint here. What the professor really means when she/he tells you to use facts is that she or he wants you to back up your claims with some evidence. It's one trick that teachers use to make sure you actually use some references when you write a paper, instead of simply offering a list of your opinions.

This may sound easy, but it's actually tough sometimes to know when you need to back up a statement with evidence and when it is OK to leave a statement unsupported.


When to Cite a Source


You should use evidence (citations) any time you make a claim that is not based on a well-known fact or common knowledge.


  • You make a claim that could be challenged.
  • You quote somebody.
  • You make a specific claim that is not common knowledge.
  • You paraphrase information from a source (give the meaning but change the wording).
  • Offer an authoritative (expert) opinion.
  • You got an idea from somebody else, even through email or conversation.


Examples of Claims You Should Support


  • Hot water can freeze faster than cold water.
  • Poodles are friendlier than Dalmatians.
  • American Chestnut trees are nearly extinct.
  • Eating while driving is more dangerous than talking on the cell phone while driving.
  • Thomas Edison invented a vote counter.



When You Don't Need to Cite a Source


Examples of Common Knowledge or Well-Known Facts

Common knowledge is basically a fact that practically everyone knows, like the fact that George Washington was a U.S. president.



  • Bears hibernate in the winter.
  • Fresh water freezes at 32 degrees F.
  • Many trees shed their leaves in the fall.


A well-know fact is something that many people know, but it is also something that a reader could look up easily if he/she didn't know.


  • It's best to plant flowers in the early spring.
  • Holland is famous for its tulips.
  • Canada has a multilingual population.


A good rule of thumb for any writer is to go ahead and use a citation when you're not certain whether or not is is necessary. The only risk in doing this is littering your paper with unnecessary citations that will drive your teacher crazy. Simply trust your own judgment.

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