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This paper aims to clarify the meaning, and explain the utility, of the case study method, a method often practiced but little understood. A "case study," I argue, is best defined as an intensive study of a single unit with an aim to generalize across a larger set of units. Case studies rely on the same sort of covariational evidence utilized in non-case study research. Thus, the case study method is correctly understood as a particular way of defining cases, not a way of analyzing cases or a way of modeling causal relations. I show that this understanding of the subject illuminates some of the persistent ambiguities of case study work, ambiguities that are, to some extent, intrinsic to the enterprise. The travails of the case study within the discipline of political science are also rooted in an insufficient appreciation of the methodological tradeoffs that this method calls forth. This paper presents the familiar contrast between case study and non-case study work as a series of characteristic strengths and weaknesses-affinities-rather than as antagonistic approaches to the empirical world. In the end, the perceived hostility between case study and non-case study research is largely unjustified and, perhaps, deserves to be regarded as a misconception. Indeed, the strongest conclusion to arise from this methodological examination concerns the complementarity of single-unit and cross-unit research designs.
The case study occupies a vexed position in the discipline of political science. On the one hand, methodologists generally view the case study method with extreme circumspection (Achen and Snidal 1989; King, Keohane, and Verba 1994; Lieberson [1991] 1992,1994; Njolstad 1990). A work that focuses its attention on a single example of a broader phenomenon is apt to be described as a "mere" case study.
At the same time, the discipline continues to produce a vast number of case studies, many of which have entered the pantheon of classic works (Allen 1965; Allison 1971; Dahl 1960; Johnson 1983; Kaufman 1960; Lazarsfeld, Berelson, and Gaudet 1948; Lijphart 1968; Pressman and Wildavsky 1973). Judging by recent scholarly output, the case study method retains considerable appeal, even among scholars in research communities not traditionally associated with this style of research-e.g., among political economists and quantitatively inclined political scientists (Acemoglu, Johnson, and Robinson 2003; Bates et...