External Treatment Effects and Program Implementation Bias

This paper discusses the definition and identification of external treatment effects and experimental designs capable of detecting these effects. External effects occur when the outcome of a given individual is affected by the treatment assignments of other individuals. The paper argues that two-stage randomization schemes, which randomize allocation of treatments across communities and randomizes the treatments themselves within communities, are useful for identifying private and external treatment effects. The importance of external treatment effects are illustrated in the context of several health economics applications: the impact of R&D subsidies, smoking prevention programs for youth, and the evaluation of HIV-prevention programs currently taking place in Africa.