Transportation Statistics and Microsimulation

Experimental Design    ◾    241

factors were both changed. e reader may verify that there is no way to

get this design by changing one variable at a time.

Figure 12.1 gives the list of aliased eects. e rst line of Figure 12.1, X1

= X2*X3, means that the main eect for background is confounded (indis-

tinguishable) from the two-way interaction eect between sign placement

and sign size. e other terms are interpreted similarly; for example, the

main eect for sign size is confounded with two-way interaction eect

between sign background and sign placement. us, for example, the

design would not let us distinguish between a big eect