EXAMPLE 6.3 Standard Deviations for Sample Means of Departure Delays

The standard deviation of the population of departure delays in Figure 6.1(a) is minutes. The delay of any single departure will often be far from the population mean. If we choose an SRS of 25 departures, the standard deviation of their mean length is

Averaging over more departures reduces the variability and makes it more likely that is close to . Our sample size of 100 departures is , so the standard deviation will be half as large: