Combining years of data is only practical for surveys that are carried out one or more times per year. Some of the surveys are conducted at periodic intervals. Although it would be possible to combine several cycles of such surveys, the length of time covered probably 10 years or more would make the results of doubtful utility. Also, SIPP uses the same households over a number of years, so that combinations of years do not provide much additional information.
The annual surveys for which combinations of years are practical are the CPS (March and monthly), NHIS, NHANES, NIS, MEPS, MCBS, and NHSDA. NHES has been omitted since there is a different emphasis in subject matter each year, so that it falls closer to periodic than annual surveys.
The plans for current NHANES implicitly assume that the detailed analyses of the survey data will be based on averages over a number of years. Each year of current NHANES is based on a representative sample of about 5,000 persons in total, far too few to provide acceptable data for the many age-sex-race/ethnicity domains NCHS considers important to study. Combinations of years will be used for analyses of these domains, probably up to 6-years for the most detailed groups. In some ways, this can be considered a model for annual averages for other surveys.