Transportation Statistics and Microsimulation

Other Inferential Procedures    ◾    149

If there is no interaction between the plotted factors, then all the

line segments will be parallel since main eects are additive. If there is

an important interaction eect, the line segments will noticeably not be

parallel, as the eects of factors dier with the combination of levels. If

the connected line segments are far from being parallel, like the one in

Figure 8.4, the assessment of the eects of the factors needs to be made

based on the interaction plots and the corresponding least-squares means,

rather than on the results from the tests of main eects. For example, the

interaction plot of ...