External validity refers to the degree to which "a causal relationship holds over variations in persons, settings, treatments, and outcomes" (Shadish, Cook, & Campbell, 2002, p. 21). In other words, external validity refers to whether the findings of research are generalizable. Campbell stressed the importance of external validity and referred to it as "situation specific wisdom," suggesting that without it, researchers will be "incompetent estimators of program impacts, turning out con¬clusions that are not only wrong, but are often wrong in socially destruc¬tive ways" (Campbell, 1984, p. 42). Some of the major threats to external validity include the following: