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Case Study

Diagnose the error in the researcher's final effect size value based on the standard conventions of Cohen's dd, and justify how they should reorganize the means or calculation steps to obtain a correct, conventional value.

Case context: A researcher is studying the efficacy of a new reading intervention. The intervention group has a post-test score mean of M=78M = 78, whereas the control group has a post-test score mean of M=84M = 84. The standard deviation across groups is SD=12SD = 12. The researcher calculates the effect size as d=788412=0.5d = \frac{78 - 84}{12} = -0.5.

Question: Diagnose the error in the researcher's final effect size value based on the standard conventions of Cohen's dd, and justify how they should reorganize the means or calculation steps to obtain a correct, conventional value.

Sample answer: The researcher's final value of 0.5-0.5 is incorrect because Cohen's dd should always be positive. To correct this, the researcher should either designate the larger mean (8484) as M1M_1 and the smaller mean (7878) as M2M_2, resulting in d=847812=0.5d = \frac{84 - 78}{12} = 0.5, or simply apply the absolute difference to the numerator: 7884=6|78 - 84| = 6, which also yields d=0.5d = 0.5.

Key points:

  • Identify that the calculated Cohen's dd of 0.5-0.5 violates the convention of always being positive.
  • Explain that the researcher should designate the larger mean (8484) as M1M_1 and the smaller mean (7878) as M2M_2.
  • Explain that taking the absolute difference in the numerator (7884|78 - 84|) corrects the issue.
  • State the corrected Cohen's dd value is 0.50.5.

Rubric: The response must explain that Cohen's dd must be positive, identify that the negative value is the issue, and explain how rearranging the means (M1=84M_1 = 84 and M2=78M_2 = 78) or taking the absolute difference resolves the error to produce 0.50.5.

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Updated 2026-05-26

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Research Methods in Psychology - 4th American Edition @ KPU

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