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A classic study by Kathy Gilliland provides a clear example of a cross-over interaction by examining how caffeine affects the verbal test scores of introverts and extraverts. The research revealed that introverts achieved better scores than extraverts when no caffeine was consumed. In stark contrast, extraverts outperformed introverts after ingesting $$4$$ mg of caffeine per kilogram of body weight. Because the performance effect of caffeine completely reversed depending on personality type, it serves as a perfect illustration of a cross-over interaction.

Example of a Cross-over Interaction

In a study on caffeine and personality, researchers utilized simple effects analyses to break down a cross-over interaction. Rather than relying on a potentially misleading main effect of caffeine averaged across all participants, a simple effects analysis allowed them to examine the specific impact of caffeine separately for introverts and extraverts. Specifically, they found that $$4$$ mg of caffeine per kilogram of body weight decreased performance for introverts but increased it for extraverts. Furthermore, they examined the effects of personality within each caffeine condition, revealing that introverts performed better without caffeine, while extraverts performed better with $$4$$ mg of caffeine. This $$2 	imes 2$$ design ultimately allowed researchers to explore $$2$$ main effects and $$4$$ distinct simple effects.

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