Though the ACS was expanded to include the population living in GQ facilities in 2006, the ACS began field testing early. The pretest in 2001 allowed the ACS to determine whether or not the new processes, type codes, and procedures would produce the desired outcome for the 2006 ACS GQ data collection operation.

In 2001, the ACS GQ operational staff and other ACS staff implemented a number of changes in the GQ operation, the greatest of which was developing an automated Group Quarters Facility Questionnaire (GQFQ). The staff developed the GQFQ based on the decennial Other Living Quarters (OLQ) questionnaire used in the 2004 Census test.

However, in order to make that questionnaire script fit with the ACS operation, the developers made some modifications, such as dropping the listing component and adding the ability to capture multiple GQ types within the GQ sampled.

Along with the introduction of an automated GQFQ, the ACS made the decision to use the revised GQ definitions planned for the 2010 Census, even though the definitions of GQ types were still evolving. The pretest used a draft version of the GQ definitions that existed at the end of November 2004. Since these definitions will continue to evolve over the next several years, the ACS needed a GQFQ that could easily adopt future revisions to the definitions.

Thus, the developers designed a flexible GQFQ. It was through this flexibility that group quarter types have been able to be added or dropped (e.g. YMCA/YWCA and hostels).

The total group quarters population in the ACS may not be comparable with the 2010 Census because there are some 2010 Census GQ types that were out of scope in the ACS. These include domestic violence shelters, soup kitchens, regularly scheduled mobile food vans, targeted non-sheltered outdoor locations, crews of maritime vessels, and living quarters for victims of natural disasters. The exclusion of these GQ types from the ACS may result in a small bias in some ACS estimates to the extent that the excluded population is different from the included population. Furthermore, only a sample of GQ facilities throughout the United States and Puerto Rico are selected for the ACS. ACS controls the GQ sample at the state level only. Therefore, for lower levels of geography, particularly when there are relatively few GQs in a geographic area, the ACS estimate of the GQ population may vary from the count from the 2010 Census.

When comparing the 2018 ACS data with previous ACS data, the data should be compared with caution at the National and State levels. It should not be compared below the State level because the weighting for the group quarters (GQ) population is not controlled below the state level. Because of this, users may observe greater fluctuations in year-to-year ACS estimates of the GQ population at sub-state levels than at state levels. The causes of these fluctuations typically are the result of either GQs that have closed or where the current population of the GQ is significantly different than the expected population as reflected on the sampling frame. Substantial changes in the ACS GQ estimates can impact ACS estimates of total population characteristics for areas where either the GQ population is a substantial proportion of the total population or where the GQ population may have very different characteristics than the total population as a whole. Users can assess the impact that year-toyear changes in sub-state GQ total population estimates have on the changes in total ACS population estimates by accessing Table B26001. Go to https://data.census.gov and enter the table number. Users should also use their local knowledge to help determine whether the year-to-year change in the ACS estimate represents a real change in the GQ population or may be the result of the lack of adequate population controls for sub-state areas.

When comparing ACS GQ data across the years when group quarters data were collected, it must be noted that beginning in 2008 military transient quarters, YMCA/YWCA and hostels were no longer in scope. These data were collected in 2006 and 2007.

Since the 2011 ACS, the GQ population has been supplemented using whole person imputation into not-in-sample GQ facilities. This increased the reliability of substate estimates for the total GQ population and characteristics of the residence population. Statelevel estimates were relatively unchanged by the new methodology.