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

Abstract

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 ζ 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 ζ 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.