Typicality is how representative an example seems of a category. A robin may be judged a more typical bird than a penguin, although both are birds. Such judgments can reflect familiarity, similarity to other members or the purposes for which a category is used; they are not the same measurement as a yes-or-no judgment of category membership.
Even a sharply defined mathematical category can produce graded psychological judgments. People can regard some odd integers as better examples than others, despite every odd integer satisfying the same membership rule. Sharon Lee Armstrong and colleagues used such cases to show that graded typicality does not, by itself, establish that a concept lacks a definition.[ref]
What some concepts might not be by Sharon Lee Armstrong and others. Experiments on well-defined concepts and discussion of typicality versus membership
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