EXAMPLE 5.8

How can we reduce the standard deviation? In the setting of Example 5.7, if we want to reduce the standard deviation of by a factor of 2, we must take a sample four times as large, n = 4 × 60, or 240. Then

For samples of size 240, about 95% of the sample means will be within twice 2.70, or 5.40 minutes, of the population mean μ.