Case Study

Explain how the sample standard deviation (SD=39.17SD = 39.17) and the sample size (n=10n = 10) are used together to determine the denominator of the tt-statistic formula, and explain what this denominator represents in the context of the study's estimate variability.

Case context: A health psychologist is examining whether university students can accurately estimate the number of calories in a chocolate chip cookie that actually contains 250 calories. A sample of 1010 students yields a mean estimate of 212.00212.00 with a standard deviation of 39.1739.17. The researcher calculates a tt-statistic of 3.07-3.07.

Question: Explain how the sample standard deviation (SD=39.17SD = 39.17) and the sample size (n=10n = 10) are used together to determine the denominator of the tt-statistic formula, and explain what this denominator represents in the context of the study's estimate variability.

Sample answer: The sample standard deviation (SD=39.17SD = 39.17) is divided by the square root of the sample size (10\sqrt{10}) to calculate the estimated standard error of the mean, which is the denominator of the tt-test formula (39.171012.39\frac{39.17}{\sqrt{10}} \approx 12.39). This denominator represents the standard error of the mean, which is the average amount of error or variation we would expect to see between different sample means and the true population mean due to chance alone.

Key points:

  • The denominator is calculated by dividing the standard deviation by the square root of the sample size (SD/nSD / \sqrt{n}).
  • This denominator represents the estimated standard error of the mean.
  • It measures the average variability or error expected between sample means by chance.

Rubric: Grading Rubric: - 2 points: Identifies the denominator calculation as dividing the standard deviation by the square root of the sample size (SD/nSD / \sqrt{n}). - 2 points: Correctly identifies this value (39.17/1039.17 / \sqrt{10} or approximately 12.3912.39) as the estimated standard error of the mean. - 2 points: Explains that it represents the expected variation or error between sample means due to random sampling variability.

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Updated 2026-05-27

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Research Methods in Psychology - 4th American Edition @ KPU

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