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2. Harms, A., Gilligan, C., 2011. Environmental Radioactivity Proficiency Test Exercise 2010, NPL Report IR 26, NPL, UK, December.
3. ISO 13528, 2005. Statistical methods for use in proficiency testing by interlaboratory comparisons, International Organization of Standardization (ISO), Geneva, Switzerland.
4. ISO Guide 35, 2006. Reference materials – General and statistical principles for certification, International Organization ofStandardization (ISO), Geneva, Switzerland.
5. General requirements for the competence of testing and calibration laboratories;ISO/IEC 17025,2005