Negative Case Selection: Justifications and Consequences for Set-Theoretic MMR

The combined usage of qualitative comparative analysis (QCA) and process tracing (PT) in set-theoretic multi-method research (MMR) holds great potential for reaching valid inferences. Established views of case selection after QCA hold that studying negative cases provides lessons about the causes of an outcome in a limited set of circumstances. In particular, recommendations focus on negative cases only if they contradict the analysis or if suitably similar positive match cases exist to leverage comparisons. By contrast, I argue that set-theoretic MMR can gain from studying negative cases even when these conditions do not hold. First, negative cases can give insights into why an outcome fails to occur. Second, they can help guard against theoretical inconsistency between explanations for the outcome and its absence. Third, they can ensure that the mechanisms producing the outcome and its absence are not too similar to be logically capable of resulting in different outcomes. Following these arguments, I recommend that studies of negative cases in set-theoretic MMR focus on failure mechanisms in carefully bounded populations, search for theoretical inconsistency among mechanisms, and focus in part on the mechanism proposed to produce the outcome.