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Case Study

Based on your understanding of null hypothesis testing, diagnose the errors in the researchers' interpretation of their pp-value.

Case context: A group of researchers conducts a study on the effect of a new cognitive behavioral therapy on anxiety levels. They report a statistically significant result with a pp-value of .02. In their discussion section, they write, 'Because our pp-value is .02, there is only a 2% probability that our null hypothesis is true. Furthermore, this means there is a 98% chance that future researchers will replicate our significant findings.'

Question: Based on your understanding of null hypothesis testing, diagnose the errors in the researchers' interpretation of their pp-value.

Sample answer: The researchers have made two errors in comprehension. First, a pp-value of .02 does not mean there is a 2% probability that the null hypothesis is true; rather, it means there is a 2% probability of observing their sample data if the null hypothesis were true. Second, it is a misconception to subtract the pp-value from 1 (1 - .02 = .98) to find the probability of replication. This ignores the crucial role of statistical power in determining whether a study will be successfully replicated.

Key points:

  • Explains that a pp-value of .02 is not the probability that the null hypothesis is true.
  • Explains that subtracting the pp-value from 1 does not yield the probability of replicating a significant result.
  • Mentions that the probability of replication also depends on statistical power.

Rubric: The student correctly explains why the researchers' interpretation of the pp-value as the probability of the null hypothesis being true is flawed, and why using 1p1 - p for replication probability is incorrect.

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Updated 2026-05-27

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Research Methods in Psychology - 4th American Edition @ KPU

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