What is common knowledge? How to avoid accidental plagiarism

As a general rule, a fact can be said to be 'common knowledge' when:

  • it is widely accessible - you may not know the total population of China, but you would be able to find the answer easily from numerous sources.
  • it is likely to be known by a lot of people
  • it can be found in a general reference resource, such as a dictionary or encyclopedia.

For example:
Pterosaurs were the flying reptiles of the dinosaur age
'Everyone' knows this, so no citation is needed.

But...
Even the largest pterosaurs may have been able to take off simply by spreading their wings whilst facing into a moderate breeze. Wilkinson, M.T., Unwin, D.M. and Ellington, C.P. (2005). High lift function of the pteroid bone and forewings of pterosaurs. Proc. R. Soc. Lond. B

Within particular disciplines, the boundaries of what is 'common' knowledge and what is 'expert' knowledge can be ambiguous, especially the further you get into your studies. If it isn't common knowledge, you'll always need to reference your source. If you are in any doubt, ask your Tutor or Supervisor.