Limitations of the application of the Horwitz equation

We revisit the basic assumptions of the Horwitz equation using the example of mycotoxin assays. Prediction intervals from the Horwitz equation often span one order of magnitude. Including this variation in calculation of Horrat values would lead to a range of values exceeding 2, which is often used as a criterion to assess interlaboratory comparisons, so we question the suitability of Horrat value for this purpose. In addition, available analytical data show significant improvement in reliability over time, which casts serious doubts on the applicability of the Horwitz equation for current analytical methods.We discuss the use of the Horwitz equation in the analytical laboratory, and conclude that it is not suitable for estimating uncertainties, as required by ISO 17025.The Horwitz equation can be a valuable summary of historical data of analytical performance. However, it should not be used as a performance criterion due to:
• shortcomings of the basic model;
• uncertainty in the values determined using it; and,
• its incompatibility with accepted methods for the determination of measurement uncertainty, as required by ISO 17025.We recommend that, instead of using the Horwitz equation, there should be a proper identification of all components of uncertainty of measurement and reasonable estimation, as stipulated by the “Guide to the Expression of Uncertainty in Measurements” (GUM).