As a result of concerns raised about the ACS estimates of total and subgroup population in some incorporated places, the ACS has adopted subcounty total population controls. This change in methodology will affect the ACS estimate of the total number of people in a particular location, but the distribution of characteristics for the most part remains comparable across years. This change applies to the 2009 ACS 1-year estimates and all future ACS 1-year, 3-year, and 5-year estimates.

The basic method is to add a procedure that controls for total population at the subcounty level while controlling for total population and additional demographic characteristics at the county level. The controls for both the subcounty and county level come from the Population Estimates Program (PEP) which is released annually by the Census Bureau. Subcounty estimates used to construct weights in the multiyear ACS products rely on an average of PEP’s subcounty estimates over the 3- or 5-year period. In general, one should expect very close agreement between the ACS estimates and the PEP estimates, but they may not be exactly equal. As with any methodological change, a small number of places may observe changes in the distribution of characteristics that are the result of the new methods. To learn more about possible impacts on characteristics, see the Research Note [PDF - <1.0 MB] on this new method.