Reducing the Risk of Data Disclosure Through Area Masking : Limiting Biases in Variance Estimation

The National Health and Nutrition Examination Survey (NHANES) is one of a series of health related programs conducted by the National Center for Health Statistics (NCHS). A unique feature of NHANES is the collection of health data by means of medical examinations carried out for a nationally representative sample of the U.S. population. Beginning in 1999, NHANES has been implemented as a continuous, annual survey. Each single year and any combination of consecutive years comprise a nationally representative sample of the U.S. population. A fourstage sample is selected for NHANES. The first stage of selection is the primary sampling unit (PSU). Within each of the selected PSUs, an average of 24 secondary sampling units (SSUs), or “segments,” consisting of census blocks or groups of census blocks are selected. Within each sampled segment, a subsample of the households is selected and screened. Within the screened households, members of particular race/ethnicity-income-sex-age subdomains are selected with pre-specified probabilities.