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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.

Example of a Main Effect

Interaction effects can be easily understood through everyday scenarios where multiple factors combine to determine a unique outcome. For example, your decision to watch a movie might depend on both the film's genre and the specific friend accompanying you. While there might be a main effect for preferring a blockbuster movie, an interaction occurs if your choice completely changes based on who is coming along. Another practical illustration is a drug interaction, where two medications may each be independently beneficial, but taking them together produces dangerous or lethal results.

Everyday Examples of Interaction Effects

In a factorial study examining driving performance with independent variables of cell phone use and time of day, which scenario describes a main effect of time of day?

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