For definitive screening designs, you typically want to fit a model that permits the possibility that two-way interactions are active. Often you also want to include pure quadratic terms in your model. You might want to assume a full second-order model, or you might want to specify an a priori model containing only certain second-order terms. The method of Efficient Effective Model Selection for DSDs using the Fit Definitive Screening platform performs well. See The Fit Definitive Screening Platform.