Assessing External Validity

Abstract In designing any causal study, steps must be taken to address both internal and external threats to its validity. Researchers tend to focus primarily on dealing with threats to internal validity. However, once they have conducted an internally valid analysis, that analysis yields an established set of findings for the specific case in question. As for the future usefulness of that result, however, what matters is its degree of external validity. In this paper we provide a formal, general exploration of the question of external validity and propose a simple and generally applicable method for evaluating the external validity of randomized controlled trials. Although our method applies only to RCTs, the issue of external validity is general and not restricted to RCTs, as shown in our formal analysis.