Validity, External/Internal

External and internal validity are crucial to the success of experimental studies in curriculum. Results of a study are said to be externally valid if inferences can be confidently made from the study's sample, either to a particular target population or across various populations and settings. By contrast, internal validity has to do with the degree to which observed differences across groups on a dependent (outcome) variable are the direct result of the manipulation of the independent (treatment) variable. Hence, external validity is concerned with generalizability of results whereas internal validity is concerned with plausibility of causal inferences.