Figure 9-19 shows that both the autocorrelation and cross correlation areinside the confidence region. Therefore, without performing any otheranalyses, you might conclude that this model is an accurate representationof the system. However, the pole-zero analysis in the Minimum DescriptionLength Criterion section showed poles outside of the unit circle. So youalready determined that this model is unstable. Thus, despite acceptableautocorrelation and cross correlation values, concluding that this model isaccurate is incorrect.Thus, if you only performed a residual analysis, you might not discover thatthis model is actually unstable. When validating a model, perform multipleanalyses to ensure the accuracy of the model.