Campbell's interest in the validity of a scientific critique of the effects of an action stemmed from his concern about establishing valid causal inferences between interventions and outcome, specifically by identifying and ruling out plausible alternative explanations of the findings. Although Campbell considered the randomized experiment to be the gold standard for evaluation research, he also advocated using rigorous quasi-experimental methods (e.g., time-series analysis and matched comparison groups) in distinguishing between program effects and effects from other factors. Campbell's advocacy of experimental and quasi-experimental study designs, however, cannot be fully understood without analyzing his acceptance of Karl Popper's view that the growth of reliable knowledge is facilitated through a critical-rational approach to the world. According to Popper, the fundamental criterion by which to judge the scientific status of a theory is "falsifiability," i.e., the thorough, unbiased testing of its claims. The author of this paper has previously argued that much of the analysis of school-based drug and violence prevention programs is not consistent with the type of critical hypothesis-testing advocated by Popper and Campbell. The current paper reviews the evidence from two evaluations of programs that have appeared on most evidence-based lists of drug and violence prevention interventions: the Seattle Social Development Project and the Life Skills Training Program, which are both school-based. The author shows how the evaluations failed to provide a scientific basis for the claim that they produced long-term desired effects. 3 figures, 4 notes, and 73 references