The emphasis on studying main effects early on in the experimentation process is supported by the empirical principle of effect hierarchy. This principle maintains that lower order effects are more likely to be important than higher order effects. For this reason, screening designs focus on identifying active main effects. In cases where higher order interactions are of interest, screening designs assume that two-factor interactions are more important than three-factor interactions, and so on. See Effect Hierarchy in Starting Out with DOE and Wu and Hamada (2009).