Each 2-year cycle of NHANES and any combination of 2-year cycles is a nationally representative sample. However, in some situations, such as when estimating the average serving size of a rarely consumed food, the sample for a single 2-year cycle is too small to produce statistically reliable estimates. The NHANES sample design makes it possible to combine data from multiple survey cycles to increase the sample size for an analysis. Increased sample size improves the statistical power, reliability, and stability of estimates for population sub-domains including racial and ethnic groups and results for rare events.
The process of combining data for multiple survey cycles or years is called appending. This is similar to adding rows to a table.
Always check the contents of each data file before appending the data files because some components or questions are not collected in every survey cycle. For example, food frequency data are collected only in the 2003-04 and 2005-06 cycles. In addition, variable names may be different from cycle to cycle, and recoded or derived variables may be added in different cycles.