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A clinical psychology researcher calculates zz scores for anxiety symptom severity in a sample of participants. They obtain the following zz scores for three participants: Participant A (z=3.00z = -3.00), Participant B (z=3.01z = -3.01), and Participant C (z=+2.99z = +2.99). Apply the formal quantitative outlier criterion to determine which of these participants, if any, have scores that are classified as outliers, and explain why.

Question: A clinical psychology researcher calculates zz scores for anxiety symptom severity in a sample of participants. They obtain the following zz scores for three participants: Participant A (z=3.00z = -3.00), Participant B (z=3.01z = -3.01), and Participant C (z=+2.99z = +2.99). Apply the formal quantitative outlier criterion to determine which of these participants, if any, have scores that are classified as outliers, and explain why.

Sample answer: Only Participant B is classified as an outlier. According to the formal definition, a score is classified as an outlier only if its zz score is strictly less than 3.00-3.00 or strictly greater than +3.00+3.00. Participant B's zz score of 3.01-3.01 is strictly less than 3.00-3.00. Participant A's score of 3.00-3.00 is not strictly less than 3.00-3.00, and Participant C's score of +2.99+2.99 is not strictly greater than +3.00+3.00.

Key points:

  • Identify Participant B as the only participant whose score is an outlier.
  • Apply the strict inequality rule (strictly less than 3.00-3.00 or strictly greater than +3.00+3.00).
  • Explain why Participant A (z=3.00z = -3.00) and Participant C (z=+2.99z = +2.99) do not qualify as outliers.

Feedback: Participant B is the only outlier because their zz score of 3.01-3.01 is strictly less than 3.00-3.00. Participant A (z=3.00z = -3.00) is not strictly less than 3.00-3.00, and Participant C (z=+2.99z = +2.99) is not strictly greater than +3.00+3.00, meaning neither falls strictly beyond the three standard deviations threshold.

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Updated 2026-05-26

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Research Methods in Psychology - 4th American Edition @ KPU

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