Medians and ranges of linear and constant coefficients of carbamazepine, fluconazole, lamotrigine and levetiracetam assay error equations obtained by developing precision profiles for all combinations of 6 or 20 samples in the three experiments conducted.

(a) unweighted linear least squares regression, (b) unweighted 2nd-order least squares polynomial regression, (c) unweighted 3rd-order least squares polynomial regression, (d) 1/x2-weighted least squares linear regression, (e) Theil’s regression, (f) Theil’s regression using the Siegel estimator. 3rd-order least squares regression was not applied in experiment 1 due to the small number of spiking levels (M = 4). (A) Linear coefficients. High/low, ratio of the highest and the lowest linear coefficients obtained in the combinatorial calculation of SD’s and, subsequently, assay error equations in each experimental setup. ND, not determined. (B) Constant coefficients (intercepts). ND, not determined. NNI, percentage proportions of non-negative intercepts obtained in the combinatorial calculation of SD’s and, subsequently, assay error equations in each experimental setup.