Case Study

Based on the logic of null hypothesis testing, what decision should the psychologists make regarding the null hypothesis? Explain the reasoning behind this decision in terms of the likelihood of the sample result when there is no true difference in the population.

Case context: A team of psychologists is studying whether a new cognitive training program improves working memory. They find that the difference in memory scores between the training group and the control group is extremely small (d=0.05d = 0.05). They evaluate this result and determine that a difference of this size is very common and likely to occur even if the training program has no real effect in the population.

Question: Based on the logic of null hypothesis testing, what decision should the psychologists make regarding the null hypothesis? Explain the reasoning behind this decision in terms of the likelihood of the sample result when there is no true difference in the population.

Sample answer: The psychologists should decide to retain the null hypothesis. Since the observed difference of d=0.05d = 0.05 is very small and is determined to be fairly likely to occur even if there were no true difference in the population, there is no strong evidence to support a real effect. Therefore, the logic of null hypothesis testing dictates retaining the null hypothesis.

Key points:

  • Identify that the researchers must retain the null hypothesis.
  • Explain that the small difference (d=0.05d = 0.05) is fairly likely to occur even if the null hypothesis is true.
  • Demonstrate comprehension that high likelihood of a sample result under the null hypothesis leads to retaining the null hypothesis.

Rubric: The student's response should be graded on whether they correctly identify the decision to retain the null hypothesis and explain that a high likelihood of the sample result under the null hypothesis (no true difference in the population) is the logical reason for retaining it.

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Updated 2026-05-27

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Research Methods in Psychology - 4th American Edition @ KPU

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