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Generally speaking, you can regard something as common knowledge if you find the same information undocumented in at least five credible sources. Additionally, it might be common knowledge if you think the information you're presenting is something your readers will already know, or something that a person could easily find in general reference sources. But when in doubt, cite; if the citation turns out to be unnecessary, your teacher or editor will tell you.Purdue Owl (http://owl.english.purdue.edu/owl/resource/589/2/)
What it is not - is other people's thougths and research
What is common knowledge to you may not be common knowledge to me - know your audience
for instance in the tutorial from Acadia University, the date of Canada's independence is listed as common knowledge - the tutorial is from a Canadian university. This may be common knowledge to them, but not to us living here in the United States.
More likely facts than theories
Barak Obama is the president of the United States.