External validity concerns whether the results of the research can be generalized to another situation, different subjects, settings, times, and so on. Key PointsHow one may go wrong in making generalizations, is one of the major threats to external validity. Usually, generalizations are limited when the cause (i.e. independent variable) is dependent upon other factors; as a result, all the threats to external validity interact with the independent variable. Aptitude-Treatment-Interaction: The sample might have some features that may interact with the independent variable causing limited generalisability, for instance, conclusions drawn from comparative psychotherapy studies mostly use specific samples (for example; volunteers, highly depressed, hardcore criminals). Situations: All the situational factors, for example, treatment conditions, light, noise, location, experimenter, timing, scope, degree of measurement, etc may limit generalizations. Pre-Test Effects: When the cause-effect relationships can only be found after the pre-tests are carried out, then, this also tends to limit the generality of the findings. Post-Test Effects: When cause-effect relationships can only be explored after the post-tests are carried out, then this can also be a cause for limiting the generalizations of the findings. Rosenthal Effects: When derivations drawn from the cause-consequence relationships cannot be generalized to other investigators or researchers. Hence sample characteristics, novelty/uniqueness of stimulus, and reactivity of the assessments are the source of threat to external validity. Therefore A, B, and E only are correct options.