For two-year cycles, the sample size may be too small to produce statistically reliable estimates for very detailed demographic sub-domains (e.g. sex-age-race/ethnicity groups) or for relatively rare events. (B. West) This is particularly true for the subsample data. The sample design for NHANES makes it possible to combine two or more "cycles" to increase the sample size and analytic options. Each two-year cycle and any combination of those two-year cycles is a nationally representative sample.
For sample size guidelines when analyzing multiple cycles see the Analytic Guidelines [PDF - 48 KB].
When combining cycles of data, it is extremely important that the user:
For the environmental chemical data the user should check the consistency of the Limit of Detection values and target age ranges across cycles. Beginning in 2003, the survey content for each two year period is held as constant as possible to be consistent with the data release cycle. In the first four years of the continuous survey this was not always the case, and some special data release and data access procedures had to be developed and used for special data files.
West, B.T., Berglund, P., and Heeringa, S.G. (2008). A Closer Examination of Subpopulation Analysis of Complex Sample Survey Data. The Stata Journal, 8(4), 520-531.