Counties are assigned to one of six levels according to the NCHS Classification Scheme, which are further collapsed to four levels in the NHIS public use files. The classification scheme "was developed for use in studying associations between urbanization level of residence and health and for monitoring the health of urban and rural residents." This classification scheme is periodically updated; please see the comparability tab for more information.

Counties designated as "Large central metro" are counties in MSAs (metropolitan statistical areas) with a population of 1 million or more that contain the entire population of the largest principal city, have their entire population in the largest principal city, or contain at least 250,000 inhabitants of any principal city of the MSA. "Large fringe metro" are counties in MSAs with a population of 1 million or more that do not qualify as large central metro counties. "Medium and small metro" are counties in MSAs with a population of less than 1 million. "Nonmetropolitan" are counties that are not in an MSA.