External Validity and Generalizability in Program Evaluation: Embracing Complexity.

This introduction to the second special issue of Evaluation review on external validity and generalizability opens a dialogue between different ways to think about generalizability in program evaluation. It argues that generalizability in impact evaluation fundamentally is about inferring some form of causality at a level broader than the specific circumstances of the initial study or studies from which these inferences are drawn. The question, then, is about how one apprehends causality: in other words, what is being generalized and how? The first special issue mainly relied on a counterfactual conception of causality, embodied by experimental and quasi-experimental methods, that aimed at impact measurement. The articles in this volume, drawing on mixed methods, also mobilize generative and configurational causal inferences to provide further levers of generalizability, focusing on how the impact is produced. The introduction insists on the specific input of qualitative methods in this respect, as theorized by grounded theory.