The external validity is concerned with generalization. If there is a causal relationship between the construct of the cause, and the effect, can the result of the study be generalized outside the scope of our study? Is there a relation between the treatment and the outcome? Threats to external validity concern ability to generalise experiment results outside the experiment setting. External validity is affected by the experiment design chosen, but also by the objects in the experiment and the subjects chosen. There are three main risks: having wrong participants as subjects, conducting the experiment in the wrong environment and performing it with a timing that affects the results. Examples: Interaction of selection and treatment, Interaction of setting and treatment, Interaction of history and treatment, Interaction of different treatments.

Wohlin, C., Runeson, P., Höst, M., Ohlsson, M., Regnell, B., & Wesslén, A. (2000). Introduction to Experimentation in Software Engineering. [-]