Is the z score sufficient to assess participants' performance in proficiency testing? The hidden corrective action

Proficiency testing providers, accreditation bodies and testing laboratories should be aware that a laboratory participating in a proficiency testing round might have reported a biased result despite a satisfactory performance indicated by an assessment using uniquely the z score. A complementary performance evaluation, based on the zeta score and the assessment of the measurement uncertainty, is therefore highly recommended. This work presents an intuitive graphical tool (the Naji2 plot) that combines z and zeta scores together with the reported measurement uncertainties. This tool allows a comprehensive assessment of the laboratory performance and enables to identify the need for corrective actions. The concerned laboratory should then perform a root cause analysis and investigate their bias and/or their measurement uncertainty evaluation.