1. ISO/IEC Guide 43-1 (1997) Proficiency testing by interlaboratory comparisons. Part 1. Development and operation of proficiency testing schemes. ISO, Geneva
2. ILAC Guide G13 (2000) Guidelines for the requirements for the competence of providers of proficiency testing schemes. http://www.ilac.org
3. International Organization for Standardization (1999) General requirements for the competence of testing and calibration laboratories. ISO/IEC 17025:1999. ISO, Geneva
4. Information about EPTN can be obtained from Dr Barry Tylee, Health & Safety Executive, Sheffield, UK, e-mail: barry.tylee@hsl.gov.uk
5. International Organization for Standardization (1994) Accuracy (trueness and precision) of measurement methods and results. Part 1. General principles and definition. Statistical methods for quality control, ISO 5725-1:1994, vol 2, pp 9?29