A school nurse would like to estimate the true mean amount of sleep that students at the high school get per night. To do so, she selects a random sample of 30 students and determines that the 90% confidence interval for the true mean amount of sleep that high school students get per night to be 6.5 to 7.5 hours. Which of the following would decrease the width of the interval?

A confidence interval gets narrower when the confidence level is decreased, because a lower confidence level uses a smaller critical value and therefore a smaller margin of error. For example, choosing 80% confidence instead of 90% would reduce the interval’s width. Decreasing the sample size or increasing the confidence level would make the interval wider.

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