External validity

External validity is the validity of applying the conclusions of a scientific study outside the context of that study.^[1] In other words, it is the extent to which the results of a study can generalize or transport to other situations, people, stimuli, and times.^[2][3] Generalizability refers to the applicability of a predefined sample to a broader population while transportability refers to the applicability of one sample to another target population.^[2] In contrast, internal validity is the validity of conclusions drawn within the context of a particular study.

Mathematical analysis of external validity concerns a determination of whether generalization across heterogeneous populations is feasible, and devising statistical and computational methods that produce valid generalizations.^[4]

In establishing external validity, scholars tend to identify the "scope" of the study, which refers to the applicability or limitations of the theory or argument of the study.^[2] This entails defining the sample of the study and the broader population that the sample represents.^[2]