Heather runs a successful lawn-mowing business. She would like to estimate the true mean amount of time it takes for her employees to mow a lawn. To do so, she selects a random sample of 30 customers and records the time it takes the employees to mow their lawns. The 90% confidence interval for the true mean time it takes for her employees to mow a lawn is 40–55 minutes. Which of the following would decrease the width of the interval?
Answer
A
selecting another sample
B
increasing the sample size
C
decreasing the sample size
D
increasing the confidence level
Explanation
The width of a confidence interval decreases when the sample size increases, because a larger sample generally gives a more precise estimate of the true mean. Decreasing the sample size would make the interval wider, and increasing the confidence level would also make it wider. Thus, increasing the sample size is the choice that narrows the interval.
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