Case Study

Describe what the positive cross-product (1.881.88) indicates about the first participant's scores relative to the group means, and explain what the final mean of the cross-products (+0.53+0.53) reveals about the overall relationship between sleep hours and alertness.

Case context: A student researcher is investigating the relationship between hours of sleep (XX: mean = 4.004.00, standard deviation = 1.901.90) and next-day alertness scores (YY: mean = 40.0040.00, standard deviation = 11.7811.78). A participant has an XX score of 77 (z=1.58z = 1.58) and a YY score of 5454 (z=1.19z = 1.19), yielding a cross-product of 1.881.88. Another participant has an XX score of 44 (z=0.00z = 0.00) and a YY score of 3030 (z=0.85z = -0.85), yielding a cross-product of 0.000.00. Across the entire sample, the mean of the cross-products is +0.53+0.53.

Question: Describe what the positive cross-product (1.881.88) indicates about the first participant's scores relative to the group means, and explain what the final mean of the cross-products (+0.53+0.53) reveals about the overall relationship between sleep hours and alertness.

Sample answer: The positive cross-product (1.881.88) indicates that the first participant scored above the mean on both sleep hours and alertness, resulting in two positive zz scores that multiply to a positive value. The final mean of the cross-products (+0.53+0.53) indicates a positive relationship between sleep hours and alertness, meaning that as sleep hours increase, alertness scores tend to increase as well.

Key points:

  • Positive zz scores correspond to raw values above their respective group means.
  • A positive cross-product indicates that a participant's scores deviate in the same direction relative to the means.
  • The mean of the cross-products represents Pearson's rr, with a positive value indicating a positive relationship between the variables.

Rubric: The response must describe that: 1) The positive cross-product shows the participant scored above the mean on both variables. 2) The mean of cross-products (+0.53+0.53) represents a positive correlation, indicating that higher sleep hours are associated with higher alertness scores.

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Updated 2026-05-27

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Research Methods in Psychology - 4th American Edition @ KPU

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