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Sample Size Determination: A bank manager wants to determine the percent of time tellers are working and idle

A bank manager wants to determine the percent of time that tellers are working and idle. She decides to use work sampling, and her initial estimate is that the tellers are idle 30 percent of the time. How many observations should the manager take in order to be 95 percent confident that the results will not be more than 2.5 percent away from the true result?

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We have the required sample size
N= (Z^2)p(1-p)/(E^2)

For 95% confidence Z = 2 ...

$2.19