The first issue in choosing sampling frames concerns the target population. In general, most national household population surveys routinely exclude people who are institutionalized and those homeless people who cannot be found in households or other living quarters visited during household surveys. SSA has decided to exclude from the DES the institutionalized population and the segment of the homeless population who cannot be found in households or other quarters at the time of the interview.
However, the question of including or excluding homeless people from the DES is not as straightforward as for the other household surveys. The committee recognizes the likelihood of relatively high rates of disability among homeless and institutionalized populations, and the resulting negative bias resulting from their exclusion. At the same time it has serious questions about the operational and methods issues. Can reliable information be obtained, feasibly and economically, from homeless and institutionalized populations? Techniques have been developed to locate, sample, and obtain data about each of these populations. Yet locating and screening respondents for eligibility require special efforts involving careful, and long-term planning, large amount of staff resources, considerable time, and high levels of funding. Homeless people present problems in scheduling, interviewing, and administering performance tests and medical examinations. Maintaining contact with them and getting them to participate in adequate numbers in the medical examination is also problematic. Likewise, obtaining permission from family members for the participation of people in long-term care institutions who are not able to grant permission themselves may be difficult.
The committee discussed these various issues and also reviewed a draft discussion paper prepared by Westat for the SSA (Westat, Inc., 1995a). The committee concurs with SSA that adding homeless and institutionalized populations to the sampling frame at this time would not be cost effective. Much research and testing are required to develop the necessary protocols and procedures for conducting the DES among homeless people and those living in different types of institutions. The costs of sampling and interviewing in the various types of institutions would be prohibitive. Thus, limiting the target population to the household population seems appropriate. The committee, however, urges SSA to undertake research as part of its long-term research plan leading to the inclusion of these populations in subsequent studies or a separate supplement to the DES in the near future.
The current proposal specifies the use of telephone number frames for DES. This decision by SSA appears to be primarily driven by cost considerations. The choice of sampling frame determines the nature of noncoverage error in any survey. Common choices in surveys in the United States are area frames, offering theoretically complete coverage of households and institutions; dual frame designs, combining telephone and area frames; dual frame designs combining area and institutional list frames; and telephone number frames.
If SSA uses a telephone frame as the screening tool, the DES will miss households without telephones. Approximately 5 percent of households in the United States are without telephones. Persons in households without telephones have a higher rate of disability than those in households with telephones. This rate is 17 percent for those without telephones compared with 15 percent among persons with telephones (Thornberry and Massey, 1988; LaPlante and Carlson, 1996). The availability of telephones also is negatively correlated with income. This noncoverage of persons in households with no telephones should be of particular concern with regard to the representativeness of the sample of persons with disabilities.