ABSTRACT
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.
Subject(s)
Food Analysis/methods , Reference Standards , Reproducibility of ResultsABSTRACT
The study considers data from 2 UK-based proficiency schemes and includes data from a total of 29 rounds and 43 test materials over a period of 3 years. The results from the 2 schemes are similar and reinforce each other. The amplification process used in quantitative polymerase chain reaction determinations predicts a mixture of normal, binomial, and lognormal distributions dominated by the latter 2. As predicted, the study results consistently follow a positively skewed distribution. Log-transformation prior to calculating z-scores is effective in establishing near-symmetric distributions that are sufficiently close to normal to justify interpretation on the basis of the normal distribution.
