World Politics

Qualitative analysts have received stern warnings that the validity of their studies may be undermined by selection bias. This article provides an overview of this problem for qualitative researchers in the field of international and comparative studies, focusing on selection bias that may result from the deliberate selection of cases by the investigator. Examples are drawn from studies of revolution, international deterrence, the politics of inflation, international terms of trade, economic growth, and industrial competitiveness. The article first explores how insights about selection bias developed in quantitative research can most productively be applied in qualitative studies. The discussion considers why qualitative researchers need to be concerned about selection bias, even if they do not care about the generality of their findings, and it considers distinctive implications of this form of bias for qualitative research, as in the problem of what is labeled "complexification based on extreme cases." The article then considers pitfalls in recent discussions of selection bias in qualitative studies. These discussions at times get bogged down in disagreements and misunderstandings over how the dependent variable is conceptualized and what the appropriate frame of comparison should be, issues that are crucial to the assessment of bias within a given study. At certain points it becomes clear that the real issue is not just selection bias, but a larger set of trade-offs among alternative analytic goals.