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Diagnose the error in the researcher's interpretation and calculation of the two-tailed -value. Explain what they should have done to correctly calculate the two-tailed -value based on the concept of extreme scores in both directions.
Case context: A researcher collects data from a sample of individuals and calculates a score of . In their draft report, they write: "We conducted a two-tailed test. Since the proportion of expected scores in the distribution that are or greater is , we reported our -value as ."
Question: Diagnose the error in the researcher's interpretation and calculation of the two-tailed -value. Explain what they should have done to correctly calculate the two-tailed -value based on the concept of extreme scores in both directions.
Sample answer: The researcher reported a one-tailed -value () instead of a two-tailed -value. To define "extreme" in both directions, they must combine the proportion of expected scores that are or greater () with the proportion of expected scores that are or lower (). Combining these two proportions yields the correct two-tailed -value of .
Key points:
- Identify that the researcher incorrectly reported a one-tailed -value instead of a two-tailed -value.
- Explain that "extreme in both directions" requires combining the positive tail ( or greater) and the negative tail ( or lower).
- Combine the two equal proportions ( and ) to find the correct -value of .
Rubric: The response must identify that the researcher reported a one-tailed value () instead of combining both tails. It must explain that a two-tailed test requires adding the probability of obtaining or lower to the probability of obtaining or higher, resulting in a correct -value of .
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Research Methods in Psychology - 4th American Edition @ KPU
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