Laboratory Proficiency Testing with Caltrans Reference Sample Program

Proficiency testing is used for assessing, ensuring, and improving the quality of laboratory test results using consistent samples with known characteristics. The results submitted by participating laboratories verify the accuracy of laboratory equipment, testing practices, technician competence, and accuracy of test results. As defined by 23 CFR 637 B, proficiency samples are homogeneous samples that are distributed and tested by two or more laboratories; the test results are compared to ensure that the laboratories are obtaining comparable results within an expected range. Laboratory and tester's ability to test materials and provide accurate test results are evaluated through proficiency testing with the California Department of Transportation (Caltrans) Reference Sample Program (RSP). Participation in Caltrans RSP is mandatory to maintain laboratory accreditation under the Caltrans Independent Assurance program. In recent years, Caltrans RSP has been used to conduct interlaboratory studies (ILS) whenever there is a concern regarding variability in test results both from Caltrans and the construction industry. Caltrans ILS is an applied research program that supports special studies requested by the Caltrans-Industry Pavement and Materials Partnering Committee (PMPC) and/or Caltrans Materials Engineering and Testing Services (METS) technical committees. The ILS plays an important role in quality improvements to materials testing by assisting with the development of new test methods and developing precision statements for various existing test methods. These precision statements are developed based on the averages and standard deviations of data collected from both ILS and RSP. Tests having standard deviations not meeting the expected precisions are considered for re-examination. Overall, the goal of ILS is to make data-driven decisions on test methods and potential changes to the standard specifications. This paper discusses an overview of proficiency testing with Caltrans RSP, recent RSPs with a scale of laboratory participation, and the role of RSP in conducting ILS. In addition, the paper also discusses how other state agencies can benefit from the lessons learned from Caltrans.