Transportation Statistics and Microsimulation

194    ◾    Transportation Statistics and Microsimulation

as x increases, then the assumption of constant variance is probably

not appropriate. When a nonconstant variance is suspected, the data

are typically handled by either performing weighted least squares or by

transformations, such as Box-Cox transformations. An excellent refer-

ence for handling nonconstant variance is Carroll and Rupert (1988).

Note that if the assumption is violated, this is usually not considered

a fatal error. is is because the estimated slope and intercept are still

unbiased, but unnecessarily inaccurate. e resulting population vari-

ances would be bigge ...