[Protocol for sampling of Chinese food and nutrition surveillance system (CFNSS)]

Based on the 145-site Disease Surveillance Point System (DSP) of the Ministry of Health in conjunction with 600-site Urban/Rural Household Socio-economic Survey System of State Statistic Bureau carried out by the Urban Household Socio-Economic Survey Team and Rural Household Socio-Economic Survey Team. Three strata including urban, general rural and less-developed rural were defined. Sample size of 16,000 children was designed according to the estimate of 5% underweight prevalence of children under 6 years of age. 40 sites were sampled by stratified random sampling, involving 14 urban sites, 17 general rural sites and 9 poor rural sites in 26 provinces. The total population of the 40 sites is 20.678 million, which is 1.7% of whole population of the country. 400 children under 6 years of age were sampled by Multi-stage Cluster Sampling at each site. The consistency of age distribution of children between the sample and the National Population Census data in 1990 by Goodness of Fit Test indicated the representativeness of the sample is reliable.