Sampling III

The prevalence, awareness, treatment, and control of hypertension in the general adult population of China were investigated. To ensure a representative sample of the population, the country was listed by locality (rural v urban communities) and then region (north v south). Twenty naturally occurring communities (clusters) were randomly selected, equally distributed between streets in urban areas and townships in rural areas for both the north and south. Within each cluster, equal numbers of men and women were randomly selected for each 10 year age range between 35 and 74 years.1 Which one of the following best describes the sampling method used in the above study?