When a source is needed
Material that is actually challenged by another editor requires a source or it may be removed; and anything likely to incur a reasonable challenge should be sourced to avoid disputes and to aid readers (see WP:BURDEN). In practice, this means most such statements are backed by an inline citation. In case of multiple possible references for a statement, the best reliable sources should be used.
- Quotations: Add an inline citation when quoting published material, whether within quotation marks or not, whether using direct or indirect speech. When using footnotes, the citation should be placed in the first footnote after the quotation. In-text attribution is often appropriate.
- Close paraphrasing: Add an inline citation when closely paraphrasing a source's words. In-text attribution is often appropriate, especially for statements describing a person's published opinions or words. In-text attribution is not appropriate for other forms of close paraphrasing, such as if you paraphrase "The sky is usually blue" as "The sky is often the color blue".
- Contentious statements about living people: Editors must take particular care adding biographical material about a living person to any Wikipedia page. Such material requires a high degree of sensitivity; do not leave unsourced information that may damage the reputation of living persons or organizations in articles.
- Exceptional claims: Exceptional claims in Wikipedia require high-quality reliable sources (see WP:REDFLAG):
- Surprising or apparently important claims not covered by mainstream sources;
- Reports of a statement by someone that seems out of character, embarrassing, controversial, or against an interest they had previously defended;
- Claims that are contradicted by the prevailing view within the relevant community, or which would significantly alter mainstream assumptions, especially in science, medicine, history, politics, and biographies of living persons, and especially when proponents consider that there is a conspiracy to silence them.
- Other: Opinions, data and statistics, and statements based on someone's scientific work should be cited and attributed to their authors in the text.