EXAMPLE 6.5 How Can we Reduce the Standard Deviation of the Sample Mean?

In the setting of Example 6.4, if we want to reduce the standard deviation of by a factor of 4, we must take a sample 16 times as large, , or 1600. Then

For samples of size 1600, about 95% of the sample means will be within twice 0.65, or 1.3 minutes, of the population mean .