The watershed manipulation project: Case study of an inter laboratory comparison program

Abstract

A major quality assurance challenge for interdisciplinary, multi‐institutional, environmental research projects is assessing and achieving interlaboratory comparability. As part of the US EPAs Watershed Manipulation Project, eight laboratories routinely measure chemical constituents of various field and laboratory‐derived solution samples. A quarterly interlaboratory comparison study has been designed by EPA quality assurance personnel in Corvallis, OR (USA) to assess: (1) ¿nfralaboratory precision and bias, (2) inferlaboratory precision, and (3) trends through time. Synthetic solution samples are prepared quarterly and sent via overnight express to participating laboratories. For each analyte, laboratories are requested to perform analyses in triplicate with each replicate measured under different calibrations; precision calculated from resulting data therefore includes a component of among‐batch variability. Relative bias is calculated as the percent difference between a laboratory's mean and the median of the laboratories (the supplying laboratory's value being treated equally to the other laboratories). Overall precision is expressed as the coefficient of variation using laboratory means. An ANOVA approach is also used to calculate significant differences among laboratories. Data from four quarterly evaluations are combined to examine changes in performance through time. The case study illustrates an approach that has proven effective in documenting and improving overall data quality and is adaptable to other multi‐laboratory research or monitoring endeavors.