Policy evaluation, randomized controlled trials, and external validity: A systematic review

When properly implemented, Randomized Controlled Trials (RCT) can achieve a high degree of internal validity. Yet, if an RCT is to inform policy interventions that extend beyond the experimental population, it is critical to establish external validity. In this paper, we first present a theoretical framework of external validity and identify the potential hazards that compromise generalizing results beyond the studied population, namely Hawthorne effects, general equilibrium effects, specific sample problems, and special care in the provision of the randomized treatment. Second, we reviewed all RCTs published in leading economic journals between 2009 and 2014 and scrutinized the way they deal with external validity. Based on a set of objective indicators, we find that many published RCTs do not discuss hazards to external validity and do not provide the information that is necessary to assess potential problems. Apparently, external validity is not an important matter of concern during the peer review process. To conclude, we call for a more systematic approach to report the results of RCTs, including external validity dimensions.