External validity refers to the accuracy of scientific results when generalized beyond the laboratory or survey situation to the real world. If it is thought that the researcher could not expect to find confirmation of research results in the life of the community, the results would be said to be externally invalid. A study that readily allows its findings to generalise to the population at large has high external validity. To the degree that we are successful in eliminating confounding variables within the study itself is referred to as Internal validity. External validity and internal validity are not all-or-none, present-or-absent dimensions of an experimental design. Validity varies along a continuum from low to high. Improving the ecological validity of an experiment will improve the external validity.
Theory Testing, Genderalization, and the Problem of External Validity - Lucas, Jeffrey. Presented at the annual meeting of the American Sociological Association. Abstract: External validity refers to the generalization of research findings, either from a sample to a larger population or to settings and populations other than those studied. While definitions vary, discussions generally agree that experiments are lower in external validity than other methodology. Further, external validity is widely treated as an issue to be addressed through methodological procedures.
When testing theories, all measures are indirect indicators of theoretical constructs, and no methodological procedures, taken alone, can produce external validity. External validity can be assessed through determining the extent to which empirical measures accurately reflect theoretical constructs and whether the research setting conforms to the scope of the theory under test. In these ways, the assessment of external validity relies on an examination of the interplay between theory and methods.
Ecological validity is generally confused with external validity. Though ecological validity and external validity are closely related, they are independent. A study may possess external validity but not ecological validity, and vice-versa. But, Improving the ecological validity of an experiment will improve the external validity.