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A central challenge in qualitative research is selecting the "negative" cases (e.g., nonrevolutions, nonwars) to be included in analyses that seek to explain positive outcomes of interest (e.g., revolutions, wars). Although it is widely recognized that the selection of negative cases is consequential for theory testing, methodologists have yet to formulate specific rules to inform this selection process. In this paper, we propose a principle-the Possibility Principle-that provides explicit, rigorous, and theoretically informed guidelines for choosing a set of negative cases. The Possibility Principle advises researchers to select only negative cases where the outcome of interest is possible. Our discussion elaborates this principle and its implications for current debates about case selection and strategies of theory testing. Major points are illustrated with substantive examples from studies of revolution, economic growth, welfare states, and war.
"I see nobody on the road," said Alice.
"I only wish I had such eyes," the King remarked, in a fretful tone.
"To be able to see Nobody! And at that distance, too!"
Lewis Carroll
(Through the Looking Glass)
Where and when do "nonsocial revolutions" occur? Certainly the United States in 1900 qualifies, but Skocpol (1979) never considered this case in her famous study of social revolutions. Nor did she choose to analyze Canada in 1890, Australia in 1950, or most of the millions of nonsocial revolutions that have occurred in world history. Instead, she selected a sample of "negative cases"1 that she regarded as relevant and appropriate for testing her theory of social revolution. In qualitative research, most analysts must-like Skocpol-select a set of negative cases to test their theories. However, the rules for choosing and justifying a set of cases defined by the occurrence of a nonevent are far from straightforward.
Intuitively, most qualitative analysts would claim that the United States in 1900 is not relevant or informative for testing theories of social revolution. Does this therefore mean that the case can be legitimately ignored when testing a theory of social revolution? Philosophers have puzzled over this question for half a century in the form of the "ravens paradox" (Hempel 1945). In this example, the hypothesis under consideration is the proposition that "all ravens are black." The positive cases that clearly support the hypothesis are black things that...