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Case Study

Diagnose the methodological error in Dr. Smith's statement. Analyze how the elements of their quote reflect the most common misinterpretation of the pp value, and explain what the 1% actually calculates in the context of their research data.

Case context: Dr. Smith conducts a psychological study on memory retention and sleep, obtaining a pp value of .01. In an interview with a science journalist, Dr. Smith says, 'This means there is only a 1% probability that my results occurred by chance, and therefore a 1% probability that the null hypothesis is true.' The journalist plans to publish this quote verbatim in a popular science magazine.

Question: Diagnose the methodological error in Dr. Smith's statement. Analyze how the elements of their quote reflect the most common misinterpretation of the pp value, and explain what the 1% actually calculates in the context of their research data.

Sample answer: Dr. Smith is making the most common misinterpretation of the pp value. By stating there is a '1% probability that the null hypothesis is true' and a '1% probability that my results occurred by chance,' they are fundamentally misunderstanding the statistic. The 1% does not calculate the probability of the null hypothesis itself. Instead, the pp value calculates the probability of obtaining Dr. Smith's specific sample data about memory and sleep, operating under the assumption that the null hypothesis is already true.

Key points:

  • Diagnoses the error as the most common misinterpretation of the pp value.
  • Analyzes the quote to show the mistaken equation of pp with the probability of the null hypothesis being true or occurring by chance.
  • Contrasts the error with the correct definition: calculating the probability of obtaining the specific sample data.
  • Notes that the correct calculation assumes the null hypothesis is already true.

Rubric: Full credit requires breaking down the quote, explicitly labeling it as the common misinterpretation, and correctly contrasting the mistaken 'probability of the null hypothesis' with the actual 'probability of the specific sample data'.

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Updated 2026-05-27

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Research Methods in Psychology - 4th American Edition @ KPU

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