Analysis of 163 disc diffusion data sets, 115 for bacterial species groups and 48 for types strains, published by EUCAST, was used to optimize the setting of the parameters of a standardized protocol for normalized resistance interpretation of these data. The standard deviations of the normalized distributions of these data sets, calculated using this standardized protocol, were shown to be independent of the means of their respective distributions. These standard deviations could, therefore, be used as a metric to quantify the precision of disc diffusion data sets. The median value of the standard deviations for 115 EUCAST data sets produced in multiple laboratory studies of bacterial species groups was 2.3mm and that for the 48 EUCAST data sets produced in multiple laboratory studies of type strains was 1.5mm. It is argued that this standardized method for estimating the precision of disc diffusion data provides a tool by which individual laboratories can assess the quality of the disc diffusion data they produce.