Common knowledge is information that is widely known and accepted by many people. The use of such information without citation does not constitute plagiarism.
Common knowledge can be easily located in a number of sources and usually cannot be disputed.
Common knowledge may differ from one academic field to another. Established principles in a profession may be considered common knowledge within that particular field of study.
In a health-related article, researchers working in medicine and health affairs could name tobacco as the leading cause of preventable death in the US without a citation because this fact is common knowledge in their field. However, a first-year health sciences student may not know this statistic and would need to cite the source of the information.
In this health-related article, the author, a researcher, may make such a statement without citing a source. However, a first-year health sciences student may not know this statistic and would need to cite the source of the information.
When in doubt about whether or not information is common knowledge, CITE YOUR SOURCE.