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Case Study

Based on the definition of percentile rank, justify why the participant's interpretation is incorrect and explain what the 85th85^{th} percentile rank actually represents regarding their performance.

Case context: A researcher administers a standardized measure of cognitive ability to a large sample of participants. She informs a participant that their raw score corresponds to the 85th85^{th} percentile rank. The participant interprets this to mean that they answered 85%85\% of the test items correctly.

Question: Based on the definition of percentile rank, justify why the participant's interpretation is incorrect and explain what the 85th85^{th} percentile rank actually represents regarding their performance.

Sample answer: The participant's interpretation is incorrect because a percentile rank is not an absolute measure of the percentage of correct answers. Instead, it describes a relative location within the distribution. A percentile rank of 8585 actually means that 85%85\% of all scores in the distribution were strictly lower than the participant's score.

Key points:

  • Identifies that percentile rank describes relative location, not an absolute test score percentage.
  • Diagnoses the participant's error by contrasting percentage correct with percentile rank.
  • Correctly defines the 85th percentile as meaning 85% of scores in the distribution are strictly lower.
  • Demonstrates comprehension of the standardized comparison function of percentile ranks.

Rubric: A full credit response must explicitly diagnose the error by differentiating an absolute percentage of correct answers from a relative percentage of lower scores, and correctly define the 85th percentile.

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Updated 2026-05-27

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KPU

Research Methods in Psychology - 4th American Edition @ KPU

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