3 - Internal and External Validity

Summary

One of the challenges in conducting interdisciplinary work, or in attempting to communicate across disciplinary boundaries, relates to the implicit norms that infuse different fields. Much like trying to speak across cultures, it often becomes frustrating to translate or make explicit differing assumptions underlying appropriate inferential methods and strategies. Status differentials often exacerbate these divergences, privileging one set of disciplinary norms over another, so that decisions about ideal methods do not always rest entirely on the appropriateness of a particular technique for a given project.

Such differences clearly affect the implementation of experimental methodology across the fields of psychology, economics, and political science. One of the areas in which these biases inflict misunderstanding surrounds issues related to internal and external validity. In political science, concerns with external validity often border on the monomaniacal, leading to the neglect, if not the complete dismissal, of attention to the important issues involved in internal validity. In psychology, the reverse emphasis predominates. In behavioral economics, the focus depends more on the primary function of the experiment. Because both internal and external validity remain important in assessing the quality, accuracy, and utility of any given experimental design, it facilitates optimal experimental design to concentrate on attempting to maximize both, but the nature of the enterprise often requires explicit consideration of the trade-offs between them.