Wikipedia's content policies require inline citations for all direct quotations, and for any material that has been challenged, or is likely to be challenged.
Wikipedia's community uses the working definition for the word likely: "probable; having a greater-than-even chance of occurring" or "having a high probability of occurring." Many statements are likely to be challenged, and many statements are unlikely to be challenged.
- If, based on your experience, a given statement has a greater than 50% chance of being challenged in good faith, either by removal, in a discussion on the talk page, or by the addition of a  or similar tag, then you should supply an inline citation for that material. This will be the case, for example, every time you add a controversial fact to an article.
- If, based on your experience, a given statement has a less than 50% chance of being challenged, then inline citations are not required for that material.
- WP:MINREF, the four types of statements for which content policies require WP:Inline citations
- WP:When to cite, detailed guidance for Featured Articles.
- Factually accurate and verifiable, the five types of material requiring inline citations for Good Articles
- WP:Common knowledge
- WP:You don't need to cite that the sky is blue
- WP:You do need to cite that the sky is blue
- WP:Why most sentences should be cited
- "Likely". Merrian-Webster. Retrieved 5 September 2015.