Interlaboratory Studies

No matter how carefully a laboratory scrutinizes its performance with internal quality control procedures, testing against other laboratories increases confidence in a laboratory’s results and among all the laboratories involved in comparison testing. Although without independent knowledge of the value of the measurand it is possible that all the laboratories involved are producing erroneous results, it is also comforting to know that your laboratory is not too different from its peers. An interlaboratory study is a planned series of analyses of a common test material performed by a number of laboratories, with the goal of evaluating the relative performances of the laboratories, the appropriateness and accuracy of the method used, or the composition and identity of the material being tested. The exact details of the study depend on the nature of the test, but all studies have a common pattern: an organizing laboratory creates and distributes a test material that is to be analyzed to the participants in the study, and the results communicated back to the organizing laboratory. The results are statistically analyzed and a report of the findings circulated. Interlaboratory studies are increasingly popular. Ongoing rounds of interlaboratory studies are conducted by most accreditation bodies; the Key Comparison program of the Consultative Committee of the Amount of Substance (CCQM) is one such interlaboratory study (BIPM 2006). There is a great deal of literature on interlaboratory studies (Hibbert 2005; Horwitz 1995; Hund et al. 2000; Lawn et al. 1997; Maier et al. 1993; Thompson and Wood 1993), and an ISO/IEC guide for the conduct of proficiency testing studies is available (ISO/IEC 1997). There are three principal groups of studies: studies that test laboratories (proficiency tests), studies that test methods, and studies that test materials (table 5.1). Laboratories that participate in method and material studies are chosen for their ability to analyze the particular material using the given method. It is not desirable to discover any lacunae in the participating laboratories, and outliers cause lots of problems. The aim of the study is to obtain information about the method or material, so confidence in the results is of the greatest importance.