The possibility principle: Choosing negative cases in comparative research

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.