The International Association of Geoanalysts (IAG) published a protocol for the certification of reference materials in close accord with the International Organisation for Standardisation (ISO) guidelines (Kane et al. 2003). This article supplements that protocol, providing additional discussion of best approaches for pre-selecting laboratories for participation in certification projects. This discussion also makes a distinction between inter-laboratory certifications, where n = 15 is the general standard, and expert laboratory certifications, where a much smaller number of laboratories will be deemed qualified to provide data of the quality needed for certification.
Kane, Jean S.; Potts, Philip J. ; Meisel, Thomas and Wiedenbeck, Michael (2007). International Association of Geoanalysts' Protocol for the Certification of Geological and Environmental Reference Materials: A Supplement. Geostandards and Geoanalytical Research, 31(3) pp. 285–288.