ISO/IEC 17043:2010 defines Proficiency Testing (PT) as the evaluation of participating laboratory performance against pre-established criteria by means of inter-laboratory comparisons. The CORESTA/Food Analysis Performance Assessment Scheme (FAPAS) PT aims to check the accuracy of results submitted by participating laboratories, where each participant was requested to identify and quantify a list of possible agrochemical residues present in a tobacco test material. In this study, statistically sound approaches have been reviewed to evaluate measurement uncertainty in the residue testing results collected annually from the CORESTA/FAPAS PT dispatched between 2006 and 2012. The calculated z-scores were combined to preserve the identity of each individual residue data point. The relative standard error was then computed to express the percentage variation and the precision, in terms of how far a sample estimate is likely to deviate from the true and unknown population value. From the available industry PT data, several elements will be presented to express uncertainty in agrochemical residue testing results.