Case Study

Explain how the numerator (Mμ0M - \mu_0) and the denominator (SDN\frac{SD}{\sqrt{N}}) of the formula function together to standardize the difference between the sample and the population. What does the denominator conceptually represent?

Case context: A psychologist evaluates the focus scores of a sample of N=16N = 16 undergraduate students who slept for 8 hours against a national standard baseline average (μ0=50\mu_0 = 50). The sample mean is M=55M = 55 and the sample standard deviation is SD=8SD = 8. She calculates the tt statistic using the formula t=Mμ0SDNt = \frac{M - \mu_0}{\frac{SD}{\sqrt{N}}}. Before performing the final math, she explains how changes in the sample statistics would influence the outcome.

Question: Explain how the numerator (Mμ0M - \mu_0) and the denominator (SDN\frac{SD}{\sqrt{N}}) of the formula function together to standardize the difference between the sample and the population. What does the denominator conceptually represent?

Sample answer: The numerator (Mμ0M - \mu_0) measures the raw distance between the sample mean and the population mean. The denominator (SDN\frac{SD}{\sqrt{N}}) conceptually represents the standard error of the mean, which is the expected variation of sample means of size NN from the true population mean. By dividing the raw distance in the numerator by the standard error in the denominator, the formula standardizes the difference, expressing it in units of standard error to determine how far the sample mean deviates from expectations.

Key points:

  • Explain that the numerator measures the raw difference between the sample mean and the hypothetical population mean
  • Define the denominator as the standard error of the mean
  • Describe how dividing the difference by the standard error standardizes the test statistic

Rubric: The response must explain: 1) the numerator represents the raw mean difference; 2) the denominator represents the standard error of the mean (expected sampling variability); 3) division by the denominator standardizes the raw difference into standard error units.

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Updated 2026-05-27

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Research Methods in Psychology - 4th American Edition @ KPU

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