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Duck test is a humorous term for a form of inductive reasoning. This is its usual expression:
If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.
The test implies that a person can identify an unknown subject by observing that subject's habitual characteristics. It is sometimes used to counter abstruse, or even valid, arguments that something is not what it appears to be.
When I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck.
The more common wording of the phrase may have originated much later with Emil Mazey, secretary-treasurer of the United Auto Workers, at a labor meeting in 1946 accusing a person of being a communist:
I can’t prove you are a Communist. But when I see a bird that quacks like a duck, walks like a duck, has feathers and webbed feet and associates with ducks—I’m certainly going to assume that he is a duck.
The term was later popularized in the United States by Richard Cunningham Patterson Jr., United States ambassador to Guatemala during the Cold War in 1950, who used the phrase when he accused the Jacobo Arbenz Guzmán government of being Communist. Patterson explained his reasoning as follows:
Suppose you see a bird walking around in a farm yard. This bird has no label that says 'duck'. But the bird certainly looks like a duck. Also, he goes to the pond and you notice that he swims like a duck. Then he opens his beak and quacks like a duck. Well, by this time you have probably reached the conclusion that the bird is a duck, whether he's wearing a label or not.
The Liskov Substitution Principle in computer science is sometimes expressed as a counter-example to the duck test:
Similarly, the term elephant test refers to situations in which an idea or thing, "is hard to describe, but instantly recognizable when spotted".
The term is often used in legal cases when there is an issue which may be open to interpretation, such as in the case of Cadogan Estates Ltd v Morris, when Lord Justice Stuart-Smith referred to "the well known elephant test. It is difficult to describe, but you know it when you see it".
- Duck typing
- I know it when I see it
- Identity of indiscernibles
- Occam's razor
- To call a spade a spade
- Zebra (medicine) As in: “When you hear hoofbeats, don’t expect to see a zebra”. 
- Heim, Michael (2007). Exploring Indiana Highways. Exploring America's Highway. p. 68. ISBN 978-0-9744358-3-1.
- Sentinel, John (September 29, 1946). "Communist Expose The Case of the Duck". Milwaukee (WI) Sentinel.
- Immerman, Richard H. (1982), The CIA in Guatemala: The Foreign Policy of Intervention, Austin, Texas: University of Texas Press, p. 102
- Denver, Joseph; Ethel Franklin Betts (1965), Cushing of Boston: A Candid Portrait
- Platt, Suzy. Respectfully quoted. Library of Congress, Congressional Research Service. ISBN 978-0-88029-768-4. "Attributed to Richard Cardinal Cushing. Everett Dirksen and Herbert V. Prochnow, Quotation Finder, p. 55 (1971). Unverified."
- Adams, Douglas (1987). Dirk Gently's Holistic Detective Agency.
- Bailey, Derick. "SOLID Development Principles – In Motivational Pictures".
- Valuing and Judging Partners — Beyond the Elephant Test!, Edge International Review, Summer 2006
- B.Wedderburn, The Worker and the Law (3rd ed, Harmondsworth, Penguin,1986), 116.
- Catherine Barnard, The Personal Scope of the Employment Relationship, in T.Araki and S.Ouchi (eds), The Mechanism for establishing and Changing Terms and Conditions of Employment/The Scope of Labor Law and the Notion of Employees, The Japan Institute for Labour Policy and Training Report, 2004, vol.1, 131-136.
- Cadogan Estates Ltd v Morris; EWCA Civ 1671 (4 November 1998) (at paragraph 17)
- Christy, Howard Chandler; Ethel Franklin Betts (1982), The complete works of James Whitcomb Riley