A company manager wishes to estimate the mean length of time it takes company crews to

Question:

A company manager wishes to estimate the mean length of time μ it takes company crews to do certain jobs. She wants to estimate m within ±7 min with 95 % confidence. Because the value of σ, the population standard duration, is unknown, she took a preliminary sample of n = 20 jobs and found that the 20 job completion times had a standard deviation of s = 18 min. How much larger should she make her sample to obtain the desired confidence interval?

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