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6. Sample size for an interval estimate of a population mean The Illinois State Toll Highway Authority is conducting a study to estimate the mean one-way distance to work for tollway commuters. The project manager desires a 2-mile bound on the error of estimation at 95% confidence. The population standard deviation is unknown, and a "best guess" of 21.75 miles is used as the planning value for o. Use the Distributions tool to help you answer the questions that follow. Select a Distribution Distributions 1 2 3 The z-value for a 95% confidence interval of the population mean is 1.645 . In order to satisfy the requirement of a 2-mile bound on the error of estimation, a sample size no smaller than 321 7 is needed