Evidential variety as a source of credibility for causal inference: beyond sharp designs and structural models

There is an ongoing debate in economics between the design-based approach and the structural approach. The main locus of contention regards how best to pursue the quest for credible causal inference. Each approach emphasizes one element -- sharp study designs versus structural models -- but these elements have well-known limitations. This paper investigates where a researcher might look for credibility when, for the causal question under study, these limitations are binding. It argues that seeking variety of evidence -- understood specifically as using multiple means of determination to robustly estimate the same causal effect -- constitutes such an alternative and that applied economists actually take advantage of it. Evidential variety is especially relevant for a class of macro-level causal questions for which the design-based and the structural approaches appear to have limited reach. The use of evidential variety is illustrated by drawing on the literature on the institutional determinants of the aggregate unemployment rate.