Assessing the performance of laboratories in large external quality assurance programs

Abstract

Interlaboratory quality assurance studies (external QA) are effective techniques to monitor the performance of laboratories analyzing environmental constituents. Environment Canada through the National Water Research Institute conducts intercomparison studies on waters, sediments and fish for inorganic and toxic organic constituents. Over 400 laboratories in Canada and the United States are involved in eight different ongoing external QA programs. Most studies include over 50 laboratories that analyze 15 to 30 different constituents in ten different samples. Data sets are addressed by non‐parametric statistics (ranking) in order to discern laboratory measurement bias (a systematic error high or low). Individual sample results that deviate significantly from interlaboratory medians are flagged either very high, high, low, or very low. In large studies (10 samples, 50 labs, 20 constituents) excellent performance is recognized when a laboratory has a very low frequency of flags and bias. Poor performance is identified when laboratories have a high frequency of bias and flags. Performance over time is analyzed through a data base management system (System 2000) and evidence clearly indicates many laboratories improving.

Keywords:

• Quality assurance
• environment
• performance
• interlaboratory studies