All surveys experience some degree of coverage error. It can take the form of under-coverage or over-coverage. Under-coverage occurs when units in the target population do not have a chance of selection into the sample; for example, addresses not listed on the frame, or people erroneously excluded from a household roster. Over-coverage occurs when units or people have multiple chances of selection; for example, addresses listed more than once on the frame, or people included on a household roster at two different sampled addresses. In general, coverage error can affect survey estimates if the characteristics of the individuals or units excluded or included in error differ from the characteristics of those correctly listed in the frame. Over- and undercoverage sometimes can be adjusted as part of the poststratification process, that is, adjusting weights to independent population control totals. Chapter 11 provides more details regarding the ACS weighting process.
The ACS uses the Master Address File (MAF) as its sampling frame, and includes several procedures for reducing coverage error in the MAF. These procedures are described below. Chapter 3 provides further details.
- Twice a year, the U.S. Census Bureau receives the U.S. Postal Service (USPS) Delivery Sequence File (DSF) that includes the addresses including a house number and street name rather than a rural route or post-office box. This file is used to update the city-style addresses on the MAF.
- The ACS nonresponse follow-up operation provides ongoing address and geography updates.
- The MAF includes address updates from special census operations.
- The Community Address Updating System (CAUS) can provide address updates (as a counterpart to the DSF updates) that cover predominately rural areas where city-style addresses generally are not used for mail delivery. CAUS was put on hold in late 2006 and is expected to be back in 2010. CAUS was put on hold because of the address canvassing operation for the 2010 Census.
The ACS Quality Measures contain housing- and person-level coverage rates (as indicators of the potential for coverage error). The coverage rates are located on the AFF for ACS data for 2007 and beyond (including all multiyear data). Coverage rates for prior years (2000 to 2006) are available on the ACS Quality Measures Web site.
Coverage rates for the total resident population are calculated by sex at the national, state, and Puerto Rico geographies, and at the national level only for Hispanics and non-Hispanics crossed by the five major race categories: White, Black, American Indian and Alaska Native, Asian, and Native Hawaiian and Other Pacific Islander. The total resident population includes persons in both housing units (HUs) and group quarters (GQ). In addition, these measures include a coverage rate specific to the GQ population at the national level. Coverage rates for HUs are calculated at the national and state level, with the exception of Puerto Rico because independent HU estimates are not available.
The coverage rate is the ratio of the ACS population or housing estimate of an area or group to the independent estimate for that area or group, multiplied by 100. The Census Bureau uses independent data on housing, births, deaths, immigration, and other categories to produce official estimates of the population and HUs each year. The base for these independent estimates is the decennial census counts. The numerator in the coverage rates is weighted to reflect the probability of selection into the sample, subsampling for personal visit follow-up, and is adjusted for unit nonresponse. The weight used for this purpose does not include poststratification adjustments (weighting adjustments that make the weighted totals match the independent estimates), since the control totals serve as the basis for comparison for the coverage rates. The ACS corrects for potential over- or under-coverage by controlling to these official estimates on specific demographic characteristics and at specific levels of geography.
As the coverage rate for a particular subgroup drops below 100 percent (indicating undercoverage), the weights of its members are adjusted upward in the final weighting procedure to reach the independent estimate. If the rate is greater than 100 percent (indicating over-coverage), the weights of its members are adjusted downward to match the independent estimates.