EXAMPLE 5.6

Standard deviations for sample means of visit lengths. The standard deviation of the population of visit lengths in Figure 5.6(a) is σ = 41.84 minutes. The length of a single visit will often be far from the population mean. If we choose an SRS of 15 visits, the standard deviation of their mean length is

Averaging over more visits reduces the variability and makes it more likely that is close to μ. Our sample size of 60 visits is 4 times 15, so the standard deviation will be half as large: