This International Standard specifies a method for the assessment of certain aspects of ambient air quality in terms of percentiles and means using the principle of stratified sampling. This is by estimating percentiles and means of the frequency distribution of measurements of ambient air quality characteristics. The application to the estimation of means, however, is restricted to cases where certain assumptions about the frequency distribution of the ambient air quality characteristic can be made using a priori knowledge or when a sufficient number of statistically independent measurements are available (see ISO 2854 and ISO 2602). The results may be used to assess ambient air quality during the period of the measurement survey. (For length of period see also ISO 7168.) By using information on the longer-term occurrence of the various strata, an assessment for a longer period can be obtained using the same database. Thus, although meteorological conditions have a profound effect on the concentration and distribution of air pollutants, stratified sampling enables results which are independent of the actual meteorological conditions prevailing during the interval of time of measurement to be calculated for a longer term.