Comparison of Reproducibility Precision on Mass Fraction in Some Interlaboratory Studies of Methods of Food Analysis

The relationship between reproducibility standard deviation and mass fraction in food analysis has been studied in compilations of statistics from collaborative trials and from proficiency tests. There was a broad tendency for both categories of statistics to follow the Horwitz function although systematic deviations from it were easily detected at both extremes of the mass fraction range (below 10−7 and above 10−2). The two compilations were found to have very similar properties over the whole range of mass fractions, that is from about 10−10 (0.1 ppb) upwards. This similarity has implications for the determination of detection limit.