Case Study

Explain what this interval indicates about the true mean calorie estimate for the population of university students, and justify whether a hypothesized population mean of 250250 is statistically significantly different from the sample mean.

Case context: A psychology researcher conducts a study on calorie estimation where a sample of 2020 university students provides a mean calorie estimate for a cookie of 200200, with a 95%95\% confidence interval ranging from 160160 to 240240.

Question: Explain what this interval indicates about the true mean calorie estimate for the population of university students, and justify whether a hypothesized population mean of 250250 is statistically significantly different from the sample mean.

Sample answer: Under the interval procedure's assumptions, repeated sampling from the same population and construction of intervals by the same method would produce intervals containing the true population mean approximately 95%95\% of the time. The fixed true mean either lies between 160160 and 240240 or does not; the confidence level describes the method's long-run coverage, rather than a 95%95\% probability that this realized interval contains the mean (NIST, Confidence Limits for the Mean). A hypothesized population mean of 250250 is statistically significantly different from the sample mean at the .05 alpha level because 250250 falls outside the 160160 to 240240 confidence interval range.

Key points:

  • Explains that 95%95\% confidence describes approximate coverage across intervals constructed by the same method in repeated sampling, under the procedure's assumptions, rather than a 95%95\% probability that the fixed true population mean lies between 160160 and 240240 in this realized interval.
  • Comprehends that the interval allows for hypothesis testing of population means.
  • Justifies that a population mean of 250250 is statistically significantly different because it is outside the interval.
  • Associates the significance testing of the boundary limits with the .05 alpha level.

Rubric: The explanation must show comprehension of 95%95\% confidence as approximate long-run coverage of the interval procedure under its assumptions, rather than a probability assigned to the fixed true population mean being inside this realized interval, and justify the significance of the hypothesized value of 250250 based on its location outside this range at the .05 level.

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Updated 2026-10-11

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Research Methods in Psychology - 4th American Edition @ KPU

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