Consider a $2 imes 2$ factorial study on driving performance involving the independent variables of cell phone use and time of day. A main effect of cell phone use is evident if, when averaging across both day and night conditions, participants without cell phones demonstrate better driving performance than those using them. Likewise, a main effect of time of day occurs if daytime driving is superior to nighttime driving, regardless of cell phone use. As another example, in a study evaluating psychotherapy, a main effect of psychotherapy length is demonstrated if longer therapy consistently yields better outcomes when averaging across all types of therapy.