multiple groups of 10 are selected, the groups are assigned to be interviewed during different months (with some exceptions, which are discussed in subsequent sections).
When the ACS sample design was first developed, the sampling rate was 3 percent of addresses annually, translating into 15 percent over 5 years. Due to budget constraints, the current annual sampling rate is around 2.2 percent, resulting in decreased reliability of the estimates. For fiscal year 2011, the Census Bureau has requested a budget increase that would allow for an increase in the sampling rate. However, because the primary concern is the reliability of the household estimates, a possibly increased budget is not expected to address challenges related to the reliability of the GQ estimates. This means that a careful look at the sample design is warranted to identify possible opportunities for increased efficiency.
As discussed above, the sample of small group quarters in a state is proportional to the number of small group quarters on the frame for that state. The sample of large group quarters is proportional to the expected number of residents in large group quarters in the state.
Because the GQ sample is not currently controlled at substate geographies, substate estimates may be highly variable, a problem that is discussed in more detail in the next section. To address this, the sampling design could be modified to exercise more control over the allocation rates at the substate level and over time. For 3-year and 5-year estimates, the sample could be required to have a minimum number of group quarters in each county over the course of the 5-year period.
Another approach would be to individualize the sample further, depending on the characteristics of small jurisdictions. For example, the lack of control over the allocation rates for smaller geographies may have a large effect on the estimates produced for a community that has 1,000 households and a correctional facility with 100 residents. According to counts from the 2000 census, places that have 10 percent or more of their population residing in group quarters represent less than 5 percent of all places in the United States. These may be the cases that would need individualized attention.
Additional control over the allocation to substate areas may be facilitated by switching from a probability proportional to size (pps) design for large group quarters to one in which strata are created on the basis of size and substate area and an equal probability sample selected within strata. This would permit the allocation to substate areas to be better controlled over time. This type of design would also simplify variance estimation, which appears to be a problem with the current design (Keathley, Navarro, and Asiala, 2010). To determine whether any efficiency would be lost by such a design, the Census Bureau could undertake a study of the effectiveness of the current pps methods. The expected population numbers in the frame are often incorrect, which reduces the efficiency of pps sampling. Consequently, the loss in precision from moving from pps to stratified, equal probability sampling may not be serious.
Recommendation 3-1: The Census Bureau should investigate the implications of controlling the ACS group quarters sample allocation at the substate level and over time to better understand how these changes would impact the precision of the estimates and the costs of the data collection at the state and substate levels.