Figure 1.

Origin of the observed total error (TEo) formula. TEo is calculated from imprecision (SD) and inaccuracy (bias) by the formula: TEo = 2SD + absolute bias, in which all elements are expressed in units of the assessed measurand. When normalized by the mean of the target value, SD becomes the coefficient of variation (CV), and the formula becomes: TEo(%) = 2CV(%) + absolute bias(%). ¹⁷ 4SD includes 95% of the results (mean ± 2SD) when values are normally distributed, which is assumed for measured replicates.