Performance expectations and acceptance criteria

Analytical chemists have long been aware that the relative standard deviation increases as the analyte concentration decreases. Pioneering work by Horwitz and the analysis of approximately 3000 values from collaborative trials has led to the establishment of an empirical function. 

This simple function is most useful in setting acceptance criteria for analytical method inter-laboratory precision. Note that the concentration, C, is relative so that 100% has a value of 1, i.e. 10 . Table 19 lists values in decreasing powers of 10. 

Other values of the Horwitz function can be calculated from the established analyte concentration value by substitution in RSD = 2 -siogC) remembering that C is the relative concentration. 

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

For example, if a collaborative trial produced a value for R5Dobserved of 9.21 for a mean analyte concentration of 0.00953% then 

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