Sampling design for the 1989 and 1995 national schistosomiasis surveys . A stratified cluster random sampling design with three strata was used. The first sampling stratum includes the seven schistosomiasis-endemic provinces (Jiangsu, Anhui, Jiangxi, Hubei, Hunan, Sichuan, and Yunnan) and one controlled province (Zhejiang). For endemic provinces, environmental and ecosystem characteristics were used to define the first sub-stratum (eco-type), and the level of prevalence defined the second sub-stratum (prevalence). A target survey population was drawn from each of the second sub-strata, with the characteristics defined in the figure. For areas that have achieved control (Zhejiang province in the 1989 and 1995 national surveys), the target survey population was drawn from two administrative villages from each historically endemic county.