Absolute Versus Relative Errors

This does not mean that the methods available today are too inaccurate to be useful. The relative accuracy for a series of related species is often much higher than the absolute accuracy attainable for any one molecule. Moreover, empirical methods can be designed to be very accurate indeed albeit with respect to a restricted set of molecular types and/or properties. Provided that the system of interest is representative of the types of molecules used for the original para-meterisation, good results can be obtained. However, we always need to be cautious of placing too much faith in computed results alone and should always cross-check theory and experiment. Together, calculation and observation make powerful complementary tools for the study of chemical systems. 

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Figure 2.11. For various combinations of n (5 10 resp. 20) and m (1, 2, resp. 3) the estimated CI(X) is plotted versus absorbance y. The left figure shows the absolute values t ixl, while the right figure depicts the relative ones, namely 100 t Sx/X in %, It is obvious that it would be inopportune to operate in the region below about 90% of nominal (in this particular case below y = 0.36 the absolute error for y = 0.36 is smaller than that for y = 0.6, but the inverse is true for the relative error, see arrows). There are three remedies increase n ox m (and costs), or reduce the calibration concentrations to shift the center of mass (x ean, ymean) below 100/0.42. At y = 0.6 and m - 1 (no replicates ) one finds X = 141.4 with a Cl of 3.39 (+2.4%, circle).

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