Transportation Statistics and Microsimulation

Graphical Methods for Displaying Data    ◾    21

correlation between population density and annual passenger miles on

public transportation, and a positive correlation between annual hours of

delay and public transportation usage. In high-density areas there may be

shorter commutes, and when there are long delays on highways, there is

an incentive to use public transportation.

e old saying “correlation is not caution” should always be kept in mind

when interpreting data. Just because two variables are correlated does not

mean that one causes the other or, as is more common, that changing one

variable will change the other. Frequently, there