External validity

External Validity is a form of experimental validity An experiment is said to possess external validity if the experiment’s results hold across different experimental settings, procedures and participants. If a study possesses external validity, its results will generalize to the larger population.

The most common loss of external validity comes from the fact that experiments using human participants often employ small samples obtained from a single geographic location. Because of this, one can not be sure that any results obtained would apply to people in other geographic locations.

External vs. Ecological Validity
External validity should not be confused with ecological validity. While external validity deals with the ability of experimental results to generalize to the “real-world” population, ecological validity is possessed when the experimental procedures resemble real-world conditions. While these forms of validity are closely related, they are independent--a study may possess external validity but not ecological validity, and vice-versa.

Qualitative research
Within the qualitative research paradigm, external validity is replaced by the concept of transferability. Transferability is the ability of research results to transfer to situations with similar parameters, populations and characteristics.