To verify the validity of the initial model, Box-Cox
transformation was implemented.
Figure 1 shows the Box-Cox transformation for the
initial model. As seen in Figure 1, the suggested
value for λ is equal to 1. In other words, the Box-Cox
does not propose a modification and the initial model
accepted as stands. The validity of the initial model
was also evaluated using residual graphs. Figure 2
shows the normal plot of the residual and the residual
versus fits for C.I. Direct Red 23. These results were
obtained with the help of MINITAB software. As can
be seen, the initial model enjoyed a good fit. Table
VIII shows the R2, R2
adj and RMSE of the model. The
plotting of the main effects for C.I. Direct Red 23 (for
S/N with "larger is better" mode) are shown in Figure