Set-theoretic Multimethod Research: The Role of Test Corridors and Conjunctions for Case Selection

Set-theoretic multimethod research (SMMR) using Qualitative Comparative Analysis (QCA) formalizes the choice of cases based on a truth table analysis. We make three recommendations for improving SMMR. First, current standards can lead to faulty case selection if causal inference on a conjunction is the goal. Case selection needs to take into account that the non-members of a conjunction might be empirically diverse and that only selected types of non-members are ideal for causal inference. Second, we formally show that cases with similar fuzzy-set memberships in a term and the outcome are the superior choice for process tracing. They minimize the expected membership in the mechanism and make it most difficult to pass a hypothesis test. Third, we propose formulas that comply with all SMMR principles and identify the best pairs of cases for analysis. We illustrate our arguments with a study of the effectiveness of sanctions against authoritarian regimes.