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(a)

(b)

Figure 1: The 3D-absorbance-response-surface representing (a) the measured multiwavelength absorption spectral dependence on pH at ${25}^{°}$C and $I=0.03$ for silybin, and (b) the 3D-residuals map after nonlinear regression performed by SQUAD (and by SPECFIT in brackets) for silybin, which exhibits 780 residuals of the residual bias $\bar{e}=3.50E-17$ ($2.48E$-$08$) being close to zero and the residual standard deviation $s(e)=1.0$1 mAU (0.88 mAU) being close to the instrumental standard deviation $s_{\text{inst}}(A)=0.30$ mAU (0.30 mAU); the residual skewness $g_1(e)=0.29$ ($-0.35$) is close to zero and indicates a symmetric distribution of residuals; the residual kurtosis $g_2(e)=2.43$ (3.21) is close to 3 and indicates a Gaussian distribution of residuals. The accuracy test of the bias proves that the bias is not significantly different from zero, (S-Plus).