Control charts to evaluate long-term performance in proficiency tests

As proficiency tests (PTs) provider, the European Union Reference Laboratory for Chemical Elements in Food of Animal Origin (EURL-CEFAO) yearly organizes exercises on the determination of chemical elements within the area of its mandate. The scheme is addressed to the National Reference Laboratories (NRLs) of the European Union Member States. Each PT is planned giving priority to the matrices and elements for which maximum levels are set in Commission Regulation No. 1881/2006 and following amendments as well as to those mainly considered in National Residues Control Plans. Samples to be distributed are prepared in the Laboratory's facilities using materials purchased on the market and often adjusting the concentration levels of the analytes around their maximum levels. In the 2006-2011 period, PTs were planned as a long-term programme pursuing the general objective of providing the NRLs with a scheme that can give them the opportunity to check and improve the performance of their analytical methods and to verify the effectiveness of any corrective actions through the repetition of the same matrix. Control charts give participants the possibility of a long-term follow-up of their performance promoting the improvement of Quality Control. To this aim, Shewhart and Cusum control charts for z-scores are prepared according to ISO 13528:2005 and updated after each PT for each laboratory. z-scores are assigned using values of sigma (pEURL-CEFAO) that correspond to the level of performance appropriate to the demand. Cusum charts are also evaluated by EURL-CEFAO in order to identify problems that may cause a bias in the measurement method.