Each subsample involves another stage of selection and therefore, a separate sample weight is calculated that accounts for that stage of selection and any additional non-response.
For the NHANES 1999–2000 and 2001–2002 survey cycles two sets of subsample weights are provided, a two-year and 4-year weight. The 2-year weights are used for analysis of a two-year cycle (1999–2000 or 2001–2002). The four-year weights should be used if these 2001–2002 data are combined with 1999–2000 data. The recommended procedure for variance estimation also requires use of stratum and Primary Sampling Unit (PSU) variables (SDMVSTRA and SDMVPSU, respectively), which are included in the demographic data file for each data release.
Subsamples of NHANES environmental chemicals are most often mutually exclusive, therefore, it is not possible to conduct an analysis where more than one analyte from different subsamples is examined together. For example, in 2005-2006, phthalates were measured in subsample “B”, but polyfluorinated compounds were measured in subsample “A”. Sometimes analytes are obtained in the same subsample and these can be analyzed together with their subsample weights. Most often these are available for analysis beginning in 2003. For example, in 2007-2008 urinary mercury and urinary arsenic were both measured in the 1/3 subsample “A”. As with all of the data files, users are encouraged to combine like subsample components across survey cycles. For example, 2005-2006 heavy metals were in subsample “A” and 2007-2008 heavy metals were in subsample “A”. This will improve the statistical reliability of the estimate.
In rare cases, there are subsamples that overlap with one another but not completely. For example, the persons who are part of the 2003-2004 1/3 subsample for urinary arsenic would also be found in the 1/2 subsample for volatile organic compounds in blood. In this situation, the data from the subsamples cannot be combined and the sample weights cannot be used. If a user attempts to combine partially overlapping subsamples the existing 1/3 and 1/2 sample weights would not be appropriate for analysis. Therefore, it is strongly advised not attempt to combine different subsamples from a single survey cycle in any analysis.