The extent to which an experiment rules out as explanations those factors that otherwise might account for the results is referred to as internal validity. Aside from evaluating the internal validity of an experiment, it is important to understand the extent to which the findings can be generalized to populations, settings, measures, experimenters, and other circumstances than those used in the original investigation. The generality of findings is referred to as external validity. Internal and external validity address central aspects of the logic of experimentation and scientific research more generally. The purpose of research is to structure the situation in such a way that inferences can be drawn about the effects of the variable of interest (internal validity) and to establish relations that extend beyond the highly specific circumstances in which the variable was examined (external validity). Internal and external validity are concepts to include in a methodological thinking tool kit. These are central to the evaluation of any study in all areas of scientific research.

Experiments are a fast-growing method in IR. What kinds of knowledge do they generate about the actual world and how does the nature of the samples affect what we find and its utility?