Recommendation 3-2: The accuracy of the measures of size used in the probability proportional to size ACS group quarters sample design should be studied. If the measures of size are seriously out of date, methods should be considered for updating the frame, as suggested in Recommendations 2-1 through 2-4.
The residents of large group quarters are subsampled in groups of 10, and some group quarters can have multiple groups of 10 in the sample. Given that group quarters provide housing and services to people with similar needs and circumstances, the intraclass correlations within group quarters are naturally high for many variables. Thus, while cost-effective, subsampling a large number of residents in a facility may be statistically inefficient. Reducing the number of persons subsampled in a facility and increasing the number of sample group quarters could improve the reliability of the estimates. This would also mean increased field costs if the number of sampled group quarters has to be increased to achieve the same level of precision of estimates. However, it is also possible that the subsample sizes could be reduced without a substantial loss in precision. If so, there may be no need to increase the number of sampled group quarters. The balance between cost and variance would have to be evaluated to determine the optimal subsample size.
A recent Census Bureau project calculated the optimal subsample size to be around four after averaging the results of calculations based on two different sets of assumptions about travel costs (Sommers and Hefter, 2010). The question can be approached in a variety of ways, particularly in terms of calculating cost savings. This is one reason why pursuing this research further is important. Future research could also take into consideration possible differences among the intraclass correlations that characterize different GQ types, given that the correlations are presumably not equally high among all of them.
Recommendation 3-3: Research on the optimal cluster size for subsampling residents in large group quarters should continue, estimating intraclass correlations for different variables and factoring in facility-level and person-level costs using a variety of approaches. The analysis should address whether the same subsample size is efficient for each GQ type and whether the size of the subsample per facility should be reduced.