METFIPS provides the original metropolitan codes for each respondent’s residence. Because both these codes and the classification of metropolitan areas change over time, METFIPS is a difficult variable to use when analyzing samples across several years. The METAREA variable uses a standard coding system across time, and although terminology for metropolitan areas and the classification of specific areas changes over time, the general concept is consistent: a metropolitan area consists of a large population center and adjacent communities that have a high degree of economic and social interaction.