Case Study

Analyze Dr. Smith's error in the calculation. How does failing to take the square root of the sample size (NN) mathematically distort the denominator of the tt statistic formula, and what impact does this specific error have on the resulting tt value? Provide the correct calculation to support your analysis.

Case context: Dr. Smith is investigating whether a new cognitive training program improves reaction times compared to the known population average of μ0=250\mu_0 = 250 milliseconds. Dr. Smith tests a sample of N=100N = 100 participants. They find a sample mean of M=240M = 240 milliseconds and a sample standard deviation of SD=40SD = 40 milliseconds. When calculating the one-sample tt-test using the formula t=Mμ0SDNt = \frac{M - \mu_0}{\frac{SD}{\sqrt{N}}}, Dr. Smith incorrectly places the sample size (NN) directly in the denominator without taking the square root, resulting in t=24025040100=100.4=25t = \frac{240 - 250}{\frac{40}{100}} = \frac{-10}{0.4} = -25.

Question: Analyze Dr. Smith's error in the calculation. How does failing to take the square root of the sample size (NN) mathematically distort the denominator of the tt statistic formula, and what impact does this specific error have on the resulting tt value? Provide the correct calculation to support your analysis.

Sample answer: By failing to take the square root of NN (100=10\sqrt{100} = 10), Dr. Smith divided the standard deviation by 100 instead of 10. This made the denominator artificially small (0.4 instead of 4). A mathematically smaller denominator inflates the final tt statistic when division occurs. The correct calculation for the denominator is 40100=4010=4\frac{40}{\sqrt{100}} = \frac{40}{10} = 4. The correct tt statistic is 2402504=104=2.50\frac{240 - 250}{4} = \frac{-10}{4} = -2.50. Dr. Smith's error resulted in an extreme, inflated tt value of -25 rather than the correct -2.50.

Key points:

  • Identifies that the square root of 100 is 10, not 100.
  • Calculates the correct denominator as 4.
  • Calculates the correct final t-statistic as -2.50.
  • Explains that the error artificially decreases the size of the denominator.
  • Concludes that an artificially small denominator mathematically inflates the absolute value of the t-statistic.

Rubric: The response must identify that the denominator should be 4, not 0.4. It must explain that dividing by N instead of the square root of N shrinks the denominator too much, thereby inflating the overall t-statistic. It must provide the correct t-value of -2.50.

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Updated 2026-05-27

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Research Methods in Psychology - 4th American Edition @ KPU

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