Case Study

Based on your understanding of interaction types, describe the exact graphical and conceptual pattern the researcher must observe to diagnose a cross-over interaction. How does this pattern differ conceptually from a spreading interaction?

Case context: A student researcher decides to replicate Kathy Gilliland's study on caffeine and verbal test scores using a 2×22 \times 2 factorial design. They recruit introverted and extraverted participants, assigning them to either a placebo (no caffeine) condition or a caffeine condition where they ingest 44 mg of caffeine per kilogram of body weight. The researcher expects to find a cross-over interaction identical to the original study.

Question: Based on your understanding of interaction types, describe the exact graphical and conceptual pattern the researcher must observe to diagnose a cross-over interaction. How does this pattern differ conceptually from a spreading interaction?

Sample answer: To diagnose a cross-over interaction, the researcher must observe that the lines representing introverts and extraverts cross each other when plotted on a graph. Conceptually, this means the effect of caffeine completely reverses depending on personality type: introverts achieve higher verbal test scores than extraverts in the no-caffeine condition, but extraverts achieve higher scores than introverts in the caffeine condition. In contrast, a spreading interaction occurs when there is an effect in one condition but weak or no effect in the other, meaning the lines diverge or 'spread' without crossing and completely reversing the direction of the difference.

Key points:

  • Comprehends that a cross-over interaction requires a complete reversal of the effect of caffeine depending on personality type.
  • Describes the graphical representation where the lines representing the groups cross each other.
  • Contrasts this with a spreading interaction where lines diverge/spread rather than crossing.
  • References the specific findings of Gilliland's study to anchor the comparison of conditions.

Rubric: The response must accurately explain that: 1) The cross-over interaction is diagnosed when the lines on the graph cross due to a complete reversal of the independent variable's effect. 2) In this study, the reversal means introverts do better without caffeine, but extraverts do better with caffeine. 3) It differs from a spreading interaction because a spreading interaction shows a difference that grows in one condition but does not reverse directions.

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Updated 2026-05-27

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Research Methods in Psychology - 4th American Edition @ KPU

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