Prior to 2015, the "big six" metro areas (Chicago, Detroit, Los Angeles, New York, Northern New Jersey, and Philadelphia) were surveyed as supplemental samples to the National survey. Thus, the microdata for these areas are part of the national PUFs. The weights on the PUFs are adjusted to national control totals. Census reweights using local control for the metropolitan publications.
Census cannot release those publication weights because there are National cases within the big six metros where we change the SMSA on the PUF to 999x to blur their location for disclosure purposes. If Census provides the internal weighting used for the publication, users would be able to tell where some of those 999x records are located. If Census releases the weights as they are, but excludes the 999x weights, users will not be able to match published totals. The PUF tabulations should be close to the report at the metro level, although subareas may be less so.
The SMSA codes included in each of the "big 6" metro areas are:
For more information, see Metropolitan Area Histories: 1973 to 2013.