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Prospective randomization is a fundamental feature of clinical trial design because this process provides a mechanism for equal distribution among treatment arms of all factors, both recognized and hidden, that might modify outcome. Although an acceptable substitute for randomization does not exist, in the area of endovascular therapy for abdominal aortic aneurysm, practical considerations often limit the use of randomization. In this regard, adjusting for case severity mix provides a mechanism to obtain some measure of confidence in comparing the outcomes of two or more treatment protocols pursued within a single clinical trial or reported by separate investigators. Relevant examples include comparing outcomes of two or more different devices undergoing separate clinical trials; analyzing results of the same technical approach reported by different investigators; and gauging the effect of an adjunctive measure …