Information can be considered "common knowledge" if multiple sources have the same information--and none of those sources have a citation for that information.
Most definitions of "common knowledge" as it applies to citation agree on two basic points:
1. Common knowledge is facts that, if asked, many (if not most) adults would know.
2. Common knowledge is also facts that, although many adults would not know by memory, are easily found in multiple reliable sources and are not disputed or controversial.
However, there are also other categories of information that would be called 'common knowledge' in certain circumstances, and in those circumstances a citation would not be required. These circumstances generally occur within two types of communities:
1. Community with a shared history, culture, language, etc.
2. Community with shared academic/disciplinary knowledge
So, a student including information about a psychological disorder (or a SWOT analysis) in a paper for an English course would need to cite the source of their information--but that same information would not need a citation in an article in a discipline-specific journal.