The sample design for NHANES makes it possible to combine two or more survey cycles to increase the sample size and analytic options. Each 2-year cycle and any combination of 2-year cycles is a nationally representative sample. However, sometimes the size of a particular sample is too small in an individual 2-year cycle to produce statistically reliable estimates. Fortunately, the NHANES sample design makes it possible to combine two or more cycles to increase the sample size and analytic options.
To produce estimates with greater statistical reliability for demographic subdomains (e.g., sex-age-race/ethnicity groups) and rare events, combining two or more 2-year cycles of the continuous NHANES is encouraged and strongly recommended. When combining cycles of data, it is extremely important that you:
For more information about determining the compatibility of datasets, please see the Locate Variables and Structure & Contents modules.
Beginning in 2003, the survey content for each 2-year period is held as constant as possible to be consistent with the data release cycle. In the first 4 years of the continuous survey (1999-2002), this was not always the case, and special data release and access procedures had to be developed for selected survey content collected in "other than 2-year" intervals. (For more details see the NHANES Data Release and Access Policy).