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State the mathematical formula for calculating Cohen's dd and explain the convention researchers use regarding the group means in the numerator to ensure the calculated effect size remains positive.

Question: State the mathematical formula for calculating Cohen's dd and explain the convention researchers use regarding the group means in the numerator to ensure the calculated effect size remains positive.

Sample answer: The formula for Cohen's dd is d=M1M2SDd = \frac{|M_1 - M_2|}{SD}. To ensure the resulting effect size is positive, researchers typically designate the larger mean as M1M_1 and the smaller mean as M2M_2, or they simply use the absolute difference between the two means in the numerator.

Key points:

  • State the formula d=M1M2SDd = \frac{|M_1 - M_2|}{SD} or describe dividing the absolute difference of group means by the standard deviation.
  • Explain that Cohen's dd values should always be positive.
  • State that the larger mean is designated as M1M_1 and the smaller as M2M_2.
  • State that the absolute difference in the numerator can be used to ensure a positive value.

Rubric: The response must correctly identify the formula for Cohen's dd (d=M1M2SDd = \frac{|M_1 - M_2|}{SD}) and clearly explain the two common methods for keeping the numerator positive: either designating the larger mean as M1M_1 and the smaller as M2M_2, or taking the absolute difference of the means.

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Updated 2026-05-26

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Research Methods in Psychology - 4th American Edition @ KPU

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