1. What is coverage error?
There are two kinds of coverage error: under-coverage and over-coverage.
Under-coverage exists when housing units or people do not have a chance of being selected in the sample.
Over-coverage exists when housing units or people have more than one chance of selection in the sample, or are included in the sample when they should not have been.
2. How does the ACS reduce coverage error?
The final ACS population estimates are adjusted for coverage error by controlling specific survey estimates to independent population controls by sex, age, race, and Hispanic origin.
The final PRCS population estimates are adjusted for coverage error by controlling specific survey estimates to independent population control by sex and age.
The ACS housing unit estimates are adjusted for coverage error by controlling the survey estimates to the independent housing unit controls for total housing units. Because of subsequent steps in the housing unit weighting process, the final ACS housing unit estimates will not agree with the independent housing unit controls.
Refer to Accuracy of the Data to learn more about this weighting procedure.
3. Why is it important to measure coverage error?
If the characteristics of under-covered or over-covered housing units or individuals differ from those that are selected, the ACS may not provide an accurate picture of the population.
4. How does the ACS measure coverage error?
The Census Bureau calculates coverage rates to measure coverage error in the ACS. 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, times 100.
Coverage rates for the total resident population are calculated by sex at the national, state, and Puerto Rico levels, and at the national level only for total 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 and group quarters. In addition, a coverage rate that includes only the group quarters population is calculated at the national level. Coverage rates for housing units are calculated at the national and state level, except for Puerto Rico because independent housing unit estimates are not available.
These rates are weighted to reflect the probability of selection into the sample, the subsampling for personal visit follow-up, and non-response. As the coverage rate drops below 100 percent, the weights of the people in the survey need greater adjustment in the final weighting procedure to reach the independent estimate. If the rate is greater than 100 percent, the ACS population estimates are downweighted to match the independent estimates.
The coverage rates for the total resident population are for the data from 2006 or later only. The 2000-2005 rates are for the comparable universes excluding the group quarters population since that population was not added to the ACS until 2006.
5. What is the independent population estimate?
The Census Bureau uses independent data on housing, births, deaths, immigration, etc. to produce official estimates of the population and housing units each year. The base for these independent estimates is the decennial census counts. The ACS, like all other household surveys, controls some of its most basic estimates to these official estimates to correct for potential over- or under-coverage.
6. What effect does the GQ coverage rate have on the quality of the ACS data?
As an example, the national GQ coverage rate for 2006 was 76.2 percent. The estimates could be biased if the 23.8 percent of the GQ population that is not covered by the survey differs from the population that was interviewed. One of the reasons for the low coverage rate was the data collection methodology. Prior to 2013, data collection in college or university housing was conducted during the entire year. The number of people in college or university housing during the months of May through August is much lower than the other months because less students attend summer sessions. The population control used in the GQ weighting methodology, however, is based on the census reference date of April 1 when most students are attending. One-third of the ACS yearly sample was allocated to these four months of May - August. An examination of the data collected during this period revealed that the population of the GQs visited in these months is not representative of the college or university housing population control. Thus, the ACS coverage rate for the summer months was artificially lower than the other eight months and as a result the overall coverage is too low. Note that if we use the eight non-summer months for college or university housing and standardize it to reflect the full-year population (weighting it up by 1.5 to account for the four summer months), the national GQ coverage rate for 2006 increases to 81.8 percent. Beginning with the 2013 data year, data collection in college or university housing is only conducted during January – April and September – December. This helps to improve the coverage of GQs and reduce any potential bias.
7. How are coverage rates calculated?