Horwitz Horrat

Precision calculated values of RSD must be in compliance with Horwitz (Horrat) values... 

Results aimed at Precision multiple results, both repeatability and reproducibility >%RSD is compared to theoretical Horwitz Horrat values Trueness Single result per test material, > calculation of Z score as measure for bias... 

The Horwitz relationship agrees with the experience of analysts and has been confirmed in various fields of trace analysis, not only in its qualitative form but also quantitatively. Thompson et al. [2004] have estimated the mathematical form of the Horwitz functiontextscHorwitz function being sH = 0.02 x0,85, or linearized, logs = 0.85 log x. The agreement of this equation is usually good and, therefore, the Horwitz functiontextscHorwitz function is sometimes used as a bench-mark for the performance of analytical methods. For this purpose, the so-called Horrat (Horwitz ratio) has been defined, Horrat = sactuai/sHy by which the actual standard deviation is compared with the estimate of the Horwitz function. Serious deviations... 

Calculated repeatability, intermediate precision, and reproducibility values can be compared with those of existing methods. If there are no methods with which to compare the precision parameters, theoretical relative reproducibility and repeatability standard deviations can be calculated from the Horwitz equation and the Horrat value (Table 5). Horwitz RSD values are reported in Table 6. Higher variability is expected as the analyte levels approach the detection limit (see below). Next to the Horwitz equation, the AOAC s Peer Verified Program proposes its own levels of acceptability of %RSD as a function of analyte concentration level [56,72]. 

Horrat value = RSDr (trial)/RSDR(Horwitz) < 2 RSDr and RSDr values according to AOAC peer-verified program... 

When the value of RSD and the mean value of the analyte concentration have been established, the Horwitz ratio, HORRAT, can be calculated. 

Precision of less than 1.5 Horwitz ratio (HORRAT) (in collaborative tests). 

The concept of HORRAT values was introduced by Horwitz etal. (1989). The HORRAT value is the ratio of RSDr obtained from the actual experimental data from intcriab-oratory data to the PRSDr calculated from the Horwitz formula ... 

An ILS provided further information on the performance of the method of McNabb et al. " Eight laboratories obtained data for eight toxins in the methanolic extracts of three contaminated mussel samples. " Some of the labs had little experience with analysis of YTX and other toxins by LC-MS. The precision estimates for YTX were repeatability 8-12% and reproducibility 15-22% with a Horwitz ratio (HorRat) of 1.3 (two extracts, levels equivalent to 2.9 and... 

Horwitz,W. and Albert, R., The Horwitz ratio (HorRat) A useful index of method performance with respect to precision. J. AOAC Int., 89, 1095, 2006. 

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