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confidence intervals are calculated by Equation 7.1, then over a very large number of repeated trials, 100(1 - as of these intervals will contain the true value of the popula- tion mean. For selected confidence levels, Table 7.2 lists corresponding values of sometimes called the reliability factor. For a 90% confidence interval, Equation 7.1 becomes the following 716454 Condutece Interval Estimation for the Mean of a Normal Destinations: Population Variance Known 273 Table 7.2 Selected Contidence Levels and Corresponding Values of a CONTEXT LEVEL 0.05 1.645 0.100 98 0.02 233 0.01 258 1.96 For a 95% confidence interval, Equation 7.1 becomes the following u60 Example 7.3 Time at the Grocery Store (Confidence interval) Suppose that shopping times for customers at a local mall are normally distributed with known population standard deviation of 20 minutes. A random sample of 64 shoppers in the local grocery store had a mean time of 75 minutes. Find the standard error, mar- gin of error, and the upper and lower confidence limits of a 95% contidence interval for the population mean