The emphasis on studying main effects early on in the experimentation process is supported by the Hierarchical Ordering Principle (Wu and Hamada, 2000). This principle contends that lower effects are more likely to be important than higher order effects. For this reason, screening designs generally assume that interactions are negligible. In cases where this assumption is not reasonable, then screening designs assume that two-factor interactions are more important than three-factor interactions, and so on.
The efficiency of screening designs also depends on the critical assumption of effect sparsity. Effect sparsity reflects the fact that real-world processes usually have only a few driving factors. Other factors are relatively unimportant. To understand the importance of effect sparsity, you can contrast screening designs to full factorial designs: