An application of combining results from multiple methods—statistical evaluation of uncertainty for NIST SRM 1508a

NIST standard reference materials (SRMs) are certified reference materials that are developed at NIST and provided to laboratories (industry, government and academia) for assessment and improvement of measurement quality. This paper details the statistical analysis related to the recertification of SRM 1508a, benzoylecgonine (cocaine metabolite) in freeze-dried urine, to incorporate new data. The recertification is based on combining measurement results from different measurement methods and time periods. Several different statistical models and corresponding estimators were considered for the certified value, its standard uncertainty and its expanded uncertainty.