External validity is the validity of generalized (causal) inferences in scientific studies, usually based on experiments as experimental validity. In other words, it is the extent to which the results of a study can be generalized to other situations and to other people.
Inferences about cause-effect relationships based on a specific scientific study are said to possess external validity if they may be generalized from the unique and idiosyncratic settings, procedures and participants to other populations and conditions. Causal inferences said to possess high degrees of external validity can reasonably be expected to apply (a) to the target population of the study (i.e. from which the sample was drawn) (also referred to as population validity), and (b) to the universe of other populations (e.g. across time and space).
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 or with idiosyncratic features (e.g. volunteers). Because of this, one cannot be sure that the conclusions drawn about cause-effect-relationships do actually apply to people in other geographic locations or without these features.
Read more about External Validity: Threats To External Validity, External, Internal, and Ecological Validity, Qualitative Research, External Validity in Experiments, The Basic Dilemma of The Social Psychologist
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