Case Study

Explain why the psychologist must square the difference between each student's study hours (XX) and the group's mean study hours (MM) before finding the mean of these differences and taking the square root. What would happen if she skipped the squaring step?

Case context: A psychologist studying student focus measures the number of hours four participants study per week. To describe the spread of these scores around the average, the psychologist decides to compute the standard deviation using the formula: SD=(XM)2NSD = \sqrt{\frac {\sum (X-M)^2}{N}}.

Question: Explain why the psychologist must square the difference between each student's study hours (XX) and the group's mean study hours (MM) before finding the mean of these differences and taking the square root. What would happen if she skipped the squaring step?

Sample answer: The psychologist must square the differences (XMX - M) because some scores will be above the mean (resulting in positive differences) and others will be below the mean (resulting in negative differences). If the differences are not squared, they will sum to zero when added together, cancelling each other out. Squaring ensures all deviations are positive, allowing a meaningful average deviation to be calculated.

Key points:

  • Deviations from the mean can be positive or negative.
  • Summing unsquared deviations results in a sum of zero because positive and negative values cancel out.
  • Squaring makes all deviations positive, allowing a meaningful sum and average to be calculated.

Rubric: The response must demonstrate an understanding that scores fall on both sides of the mean (positive and negative deviations) and that summing unsquared deviations results in a sum of zero. It must explain that squaring makes all deviations positive so they can be summed and averaged.

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Updated 2026-05-27

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Research Methods in Psychology - 4th American Edition @ KPU

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