Case Study

Diagnose the error in the researcher's interpretation and calculation of the two-tailed pp-value. Explain what they should have done to correctly calculate the two-tailed pp-value based on the concept of extreme scores in both directions.

Case context: A researcher collects data from a sample of 2525 individuals and calculates a tt score of 1.50. In their draft report, they write: "We conducted a two-tailed test. Since the proportion of expected tt scores in the distribution that are 1.50 or greater is .07, we reported our pp-value as .07."

Question: Diagnose the error in the researcher's interpretation and calculation of the two-tailed pp-value. Explain what they should have done to correctly calculate the two-tailed pp-value based on the concept of extreme scores in both directions.

Sample answer: The researcher reported a one-tailed pp-value (.07) instead of a two-tailed pp-value. To define "extreme" in both directions, they must combine the proportion of expected tt scores that are 1.50 or greater (.07) with the proportion of expected tt scores that are 1.50-1.50 or lower (.07). Combining these two proportions yields the correct two-tailed pp-value of .14.

Key points:

  • Identify that the researcher incorrectly reported a one-tailed pp-value instead of a two-tailed pp-value.
  • Explain that "extreme in both directions" requires combining the positive tail (1.50 or greater) and the negative tail (1.50-1.50 or lower).
  • Combine the two equal proportions (.07 and .07) to find the correct pp-value of .14.

Rubric: The response must identify that the researcher reported a one-tailed value (.07) instead of combining both tails. It must explain that a two-tailed test requires adding the probability of obtaining 1.50-1.50 or lower to the probability of obtaining 1.50 or higher, resulting in a correct pp-value of .14.

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Updated 2026-05-27

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Research Methods in Psychology - 4th American Edition @ KPU

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