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A psychology researcher compares the cognitive test scores of two groups. Group A has a mean score of M=45M = 45, and Group B has a mean score of M=30M = 30. The standard deviation for the scores is SD=10SD = 10. Apply the Cohen's dd formula to calculate the effect size for this study, showing your steps.

Question: A psychology researcher compares the cognitive test scores of two groups. Group A has a mean score of M=45M = 45, and Group B has a mean score of M=30M = 30. The standard deviation for the scores is SD=10SD = 10. Apply the Cohen's dd formula to calculate the effect size for this study, showing your steps.

Sample answer: Using the formula d=M1M2SDd = \frac{|M_1 - M_2|}{SD}, we substitute the values: d=453010=1510=1.5d = \frac{|45 - 30|}{10} = \frac{15}{10} = 1.5. The calculated Cohen's dd is 1.51.5.

Key points:

  • Calculate the mean difference in the numerator: 4530=1545 - 30 = 15 (or 3045=15|30 - 45| = 15).
  • Divide the mean difference by the standard deviation (1010).
  • State the final Cohen's dd value as 1.51.5.

Rubric: The student must show the calculation steps: finding the difference between means (4530=1545 - 30 = 15) and dividing by the standard deviation (1010) to arrive at the final positive Cohen's dd value of 1.51.5.

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Updated 2026-05-26

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Research Methods in Psychology - 4th American Edition @ KPU

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