Experiments in economics: External validity and the robustness of phenomena

External validity is the problem of generalizing results from laboratory to non-laboratory conditions. In this paper we review various ways in which the problem can be tackled, depending on the kind of experiment one is doing. Using a concrete example, we highlight in particular the distinction between external validity and robustness, and point out that many experiments are not aimed at a well-specified real-world target but rather contribute to a 'library of robust phenomena', a body of experimental knowledge to be applied case by case.