Since 1999, NHANES data files have been released for public use in 2-year groupings, also known as cycles. The release of NHANES data in 2-year cycles allows enough cases (observations) for analysis in each cycle. However, you may sometimes wish to combine multiple survey cycles, add additional observations, or combine different years of data files on the same variables. For instance, in order to produce estimates with greater statistical reliability, combining two or more 2-year cycles of the continuous NHANES (1999 and after) is encouraged and strongly recommended for estimates for demographic subdomains. The process of combining years is called appending. This is similar to adding rows to a table.
Check the contents of the data files before appending the data because variable names may be different from cycle to cycle and recoded or derived variables may be added in different cycles.
NHANES adds or deletes survey items from time to time. If these added or deleted variables are not relevant to your analysis, you can simply append the data files as described and use only the variables of interest for your analysis. The extra variables will not affect your analysis if you do not include them in your dataset.
When appending NHANES data you should always include the SEQN number. SEQN stands for sequence number and is a unique identifier for each observation (participant) in NHANES. Every time you extract variables from an NHANES data file, you should include the SEQN variable in your selection. Failing to do so will lead to problems if you want to sort or merge your data files at a later time.
After appending the data, you will need to check the results. You should see that all your variables of interest were included and that any variables you renamed or recoded are correct and include all the years of data.