12. The uncertainty (expressed at a 95% confidence level) of the methods used for the assessment of ambient air concentrations shall be evaluated in accordance with the CEN Guide to the Expression of Uncertainty in Measurement (ENV 13005-1999)(1), the methodology of ISO 5725:1994, and the guidance provided in the CEN Report, “Air quality – Approach to uncertainty estimation for ambient air reference measurement methods” (CR 14377:2002E). The percentages for uncertainty in the table in paragraph 11 are given for individual measurements, which are averaged over typical sampling times, for a 95% confidence interval. The uncertainty of the measurements shall be interpreted as being applicable in the region of the appropriate target value. Fixed and indicative measurements shall be evenly distributed over the year in order to avoid skewing of results.
European Committee for Standardisation (CEN) publication; (copies may be obtained from CEN at 36, Rue de Stassart, B 1050, Brussels, Belgium, http://www.cenorm.be).