TECH Brief 15 – Using the Horwitz Function for Acceptability of Precision

The assessment of precision as repeatability and within-lab reproducibility should be based on the use of the function of Horwitz as an indicator of whether the obtained precision is adequate for the concentration level measured. The Horwitz function is used to assess the performance of the data under consideration, with respect to precision. The original Horwitz equation reflects variability seen in interlaboratory studies, where results inherently show higher spread due to the diversity of labs. The modified Horwitz function (scaled by 0.67) is tailored to intra-laboratory precision, where the variability is reduced due to controlled and consistent conditions.