External Validity: A Survey-Experiment Approach
Campbell and Stanley (1966) have suggested that experimental results be evaluated from the perspectives of internal and external validity. The internal validity perspective asks how many rival hypotheses for the results of an experiment can be rejected. While Campbell and Stanley discuss more general threats, such as maturation and statistical regression, which the &dquo;true&dquo; experimental designs control for, Wiggins.(1968) extends this notion to include plausible theoretical alternatives as well. For example, if we attribute the in-