Question: Biostatistics & Epidemiology
Investigators report that for a group of patients the average diastolic blood pressure is 113 mm Hg, with a 95% confidence interval of 110 to 116 mm Hg. If the investigators instead wished to report a 99% confidence interval using the same data, which of the following would most likely be true of the new interval?
Choose one answer.
Explanation
Correct answer: B. It would be wider and still centered on 113 mm Hg
A confidence interval is centered on the sample mean and has a half-width equal to the critical value times the standard error. Raising the confidence level from 95% to 99% increases the critical value (from about 1.96 to about 2.58 standard errors) while the mean and standard error are unchanged, so the interval gets wider but stays centered on 113 mm Hg. Greater confidence requires a wider interval to be more certain it captures the true mean.
Learning objective: Explain how increasing the confidence level widens a confidence interval.
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