Metropolitan Statistical Areas (MSAs), Micropolitan Statistical Areas, Metropolitan Divisions, and Combined Statistical Areas (CSAs)
Statistical areas are defined (geographically delineated) by the Office of Management and Budget in bulletin no. 15-01 issued July 15, 2015, and the definitions are updated as new information warrants.  When OMB adds a new statistical area, BEA creates a time series for it starting in the earliest year in which data is available, even though it may not have had any urban area at the time. Similarly, when OMB changes the definition of a statistical area, BEA recreates the time series for that area, using the new definition for every year in the time series, published at the scheduled release date of the data set.
 OMB metropolitan area revision bulletins