Extraordinary claims require extraordinary evidence

"Extraordinary claims require extraordinary evidence" (sometimes shortened to ECREE),^[1] also known as the Sagan standard, is an aphorism popularized by science communicator Carl Sagan. He used the phrase in his 1979 book Broca's Brain and the 1980 television program Cosmos. It has been described as fundamental to the scientific method and is regarded as encapsulating the basic principles of scientific skepticism.

The concept is similar to Occam's razor in that both heuristics prefer simpler explanations of a phenomenon to more complicated ones. In application, there is some ambiguity regarding when evidence is deemed sufficiently "extraordinary". It is often invoked to challenge data and scientific findings, or to criticize pseudoscientific claims. Some critics have argued that the standard can suppress innovation and affirm confirmation biases.

Philosopher David Hume characterized the principle in his 1748 essay "Of Miracles". Similar statements were made by figures such as Thomas Jefferson in 1808, Pierre-Simon Laplace in 1814, and Théodore Flournoy in 1899. The formulation "Extraordinary claims require extraordinary proof" was used a year prior to Sagan, by scientific skeptic Marcello Truzzi.

Critics state that it is impossible to objectively define the term "extraordinary" and that measures of "extraordinary evidence" are completely reliant on subjective evaluation. Ambiguity in what constitutes "extraordinary" has led to misuse of the aphorism, and it is frequently invoked to discredit research dealing with scientific anomalies or any claim that falls outside the mainstream.^[2][3]