1. American Industrial Hygiene Association (AIHA). AIHA proficiency analytical testing program [Internet]. [cited 2017 Mar 13]. Available from: http://www.aihapat.org/Pages/default.aspx.
2. Health and Safety Laboratory (HSL). Proficiency testing schemes [Internet]. [cited 2017 Mar 13]. Available from: http://www.hsl.gov.uk/proficiency-testing-schemes.
3. Occupational safety and health act. Act 9434;Ministry of Employment and Labor (MOEL),2009
4. Conformity assessment – general requirements for proficiency testing;International Organization for Standardization (ISO),2010