Case Study

Based on the provided context and Cohen's guidelines, explain why the researcher is justified in describing the relationship strength as moderate (medium) and negative. Why is the strength of this relationship equal to a positive correlation of +0.31+0.31?

Case context: A developmental psychologist investigates the relationship between the number of hours infants spend in daycare per week and their attachment security scores. After collecting data from 80 families, the researcher runs a correlation analysis and obtains a Pearson's correlation coefficient of r=0.31r = -0.31. The researcher concludes that daycare hours have a moderate, negative relationship with attachment security, meaning that more hours in daycare are associated with slightly lower attachment scores.

Question: Based on the provided context and Cohen's guidelines, explain why the researcher is justified in describing the relationship strength as moderate (medium) and negative. Why is the strength of this relationship equal to a positive correlation of +0.31+0.31?

Sample answer: The researcher is justified because an rr value of 0.31-0.31 is near ±0.30\pm 0.30, which Cohen's guidelines classify as a medium (moderate) relationship. The negative sign indicates the direction of the relationship is negative (inverse), where one variable increases as the other decreases. The sign does not affect the strength of the relationship, making a correlation of 0.31-0.31 exactly as strong as +0.31+0.31.

Key points:

  • Explain that r=0.31r = -0.31 is classified as a medium/moderate relationship because it is close to ±0.30\pm 0.30.
  • Explain that the negative sign represents a negative/inverse direction of the relationship.
  • Explain that the strength of a correlation coefficient is independent of its sign, making 0.31-0.31 and +0.31+0.31 equally strong.

Rubric: The answer must demonstrate comprehension of Cohen's guidelines by: 1) Explaining that 0.31-0.31 is near the ±0.30\pm 0.30 threshold for a medium relationship. 2) Explaining that the negative sign denotes the inverse direction of the association. 3) Explaining that the sign is independent of strength, so 0.31-0.31 and +0.31+0.31 are equally strong.

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Updated 2026-05-26

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Research Methods in Psychology - 4th American Edition @ KPU

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