For a fixed number of runs, when can designs have clear main effects or clear two-factor interactions (in brief, 2fi' s)? This paper gives the maximum value of n in sn m designs containing clear main effects or clear 2fi' s, where s is any prime or prime power. It is a generalization of the related results in Chen and Hedayat (J. Statist. Plann. Inference 75 (1998) 147-158) for two-level designs. It is further concluded that the weak minimum aberration designs have a maximum number of clear main effects for two-level designs. A collection of designs containing most clear main effects or clear 2fi' s for 16, 32, 27, and 81 runs is given.