Figure 2. Twenty-one cities of Guangdong province were applied by a multi-stage stratified and cluster random sampling for sample selection in each survey.

Guangdong province, including one capital-city (Guangzhou city) and twenty middle and small-sized cities, was divided into three population strata: a) the core area of the capital city, b) prefectures of other urban areas and c) prefectures of rural areas. At least two districts in each of the three population strata were randomly selected.