Multi-component out-of-specification test results: a case study of concentration of pesticide residues in tomatoes

A metrological approach is used for investigating multi-component out-of-specification (OOS) test results of pesticide residues concentration in tomatoes. As a case study, 169 test results were obtained in Israel in 2009. Five of the test results were OOS test results exceeding the national legal maximum residue limits (MRL). Only one of them was classified definitely (with more than 0.99 confidence) as caused by a farmer’s/producer’s problem. The other four OOS test results were probably metrologically related, i.e., compatible with MRL when considering the measurement uncertainty associated with the test results. A new parameter—the ratio of a test result to MRL—was proposed for analysis of tomatoes monitoring multi-residue data as a common statistical sample from the same population for different pesticide residues. Weibull distribution was found adequate for modeling the empirical distribution of the parameter values. Probability of future OOS test results was estimated, and global risks of farmer/producer and consumer/buyer were evaluated. Acceptance limits for the test results, such as “warning and action lines” in quality control charts, were calculated by taking into account the measurement uncertainties.