CONTROL can be used to link records across datasets and between survey years. However, in the metro data for 1998, 2002, 2004, 2007, and 2009 CONTROL must be combined with SMSA to uniquely identify housing units and link records. Beginning with 2015, the first three 1’digits are 110 for National sample records, and 210 for Metropolitan sample records. The last five digits are sequentially numbered beginning with 00001.
For more about geography in the PUF, see the Public Use File Geography: 1985 to 2013 documentation.