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 be generalized to and across other situations, people, stimuli, and times.[2] In contrast, internal validity is the validity of conclusions drawn within the context of a particular study. Because general conclusions are almost always a goal in research, external validity is an important property of any 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.