Case Study

Explain why the assistant's assertion is incorrect, how the actual shape of the distribution is determined, and calculate the specific degrees of freedom value for this sample.

Case context: A health psychology researcher is comparing the average daily caffeine intake of a sample of 12 college students to a recommended limit using a one-sample tt-test. A student assistant suggests that the shape of the tt distribution is fixed and will be the same regardless of how many students are recruited for the study.

Question: Explain why the assistant's assertion is incorrect, how the actual shape of the distribution is determined, and calculate the specific degrees of freedom value for this sample.

Sample answer: The assistant's assertion is incorrect because the shape of a tt distribution is not fixed; instead, it is determined by the degrees of freedom (dfdf). For a one-sample tt-test, the degrees of freedom are calculated as the sample size minus one (df=N1df = N - 1). For this sample of 12 students, the degrees of freedom are 121=1112 - 1 = 11. This value of 1111 determines the exact shape of the tt distribution used for the analysis.

Key points:

  • The degrees of freedom (dfdf) determine the exact shape of the tt distribution.
  • The formula for a one-sample tt-test is df=N1df = N - 1.
  • A sample size of N=12N = 12 results in df=11df = 11, which dictates the distribution's shape.

Rubric: The response should explain that the shape of the tt distribution depends on the degrees of freedom rather than being static, state the formula df=N1df = N - 1, and correctly calculate that the degrees of freedom for this case study are 1111.

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Updated 2026-05-26

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Research Methods in Psychology - 4th American Edition @ KPU

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