5Randomized comparative experiments are designed to give good evidence that differences in the treatments actually cause the differences we see in the response. The logic is as follows:Random assignment of subjects forms groups that should be similar in all respects before the treatments are applied. Comparative design ensures that influences other than the experimental treatments operate equally on all groups.Therefore, differences in average response must be due either to the treatments or to the play of chance in the random assignment of subjects to the treatments.Principles of experimental design:The basic principles of statistical design of experiments are1.Control the effects of lurking variables on the response, most simply by comparing two or more treatments.2.Randomize—use chance to assign subjects to treatments.3.Use enough subjects in each group to reduce chance variation in the results.
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