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Case Study

Analyze the psychologist's proposed null and alternative hypotheses. Identify the errors in their formulation of hypotheses for a standard one-sample tt-test, justify why these statements are incorrect based on the definition of a one-sample tt-test, and write the correct null and alternative hypotheses using mathematical notation.

Case context: A health psychologist is investigating whether a new mindfulness app affects the average daily stress levels of users. The standardized scale used has a known general population neutral mean score of μ0=25\mu_0 = 25. The psychologist collects data from 40 participants using the app and wants to run a statistical test to see if their stress scores differ from the general population mean. In their write-up, the psychologist proposes: 'The null hypothesis is that the app will decrease stress, meaning μ<25\mu < 25, and the alternative hypothesis is that the app will increase stress, meaning μ>25\mu > 25.'

Question: Analyze the psychologist's proposed null and alternative hypotheses. Identify the errors in their formulation of hypotheses for a standard one-sample tt-test, justify why these statements are incorrect based on the definition of a one-sample tt-test, and write the correct null and alternative hypotheses using mathematical notation.

Sample answer: The psychologist's hypotheses are incorrect because a standard one-sample tt-test evaluates whether a population mean differs from a comparison mean, which requires a non-directional alternative hypothesis and a null hypothesis of exact equality. The psychologist incorrectly formulated the null hypothesis as a directional decrease (μ<25\mu < 25) and the alternative as a directional increase (μ>25\mu > 25). The null hypothesis must assume that the true population mean (μ\mu) is exactly equal to the hypothetical comparison mean (μ0=25\mu_0 = 25), written as μ=μ0\mu = \mu_0. The alternative hypothesis must state that the population mean differs from the comparison mean, written as μμ0\mu \neq \mu_0.

Key points:

  • Identify that the null hypothesis must assume exact equality between the population mean and comparison mean (μ=μ0\mu = \mu_0).
  • Identify that the alternative hypothesis must state that the population mean differs from the comparison mean (μμ0\mu \neq \mu_0).
  • Justify why directional inequalities (μ<25\mu < 25 and μ>25\mu > 25) are incorrect for a standard one-sample tt-test.
  • Provide correct mathematical notation for both hypotheses (μ=25\mu = 25 and μ25\mu \neq 25).

Rubric: To earn full credit, the response must: 1. Identify that the null hypothesis incorrectly proposes a directional inequality (μ<25\mu < 25) instead of assuming exact equality (μ=μ0\mu = \mu_0). 2. Identify that the alternative hypothesis incorrectly proposes a directional increase (μ>25\mu > 25) instead of stating that the population mean differs from the comparison mean (μμ0\mu \neq \mu_0). 3. Justify that a standard one-sample t-test requires testing for a difference in either direction. 4. Provide the correct notation for both hypotheses: null (μ=25\mu = 25) and alternative (μ25\mu \neq 25).

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Updated 2026-05-27

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Research Methods in Psychology - 4th American Edition @ KPU

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