result of changes in the disability decision process. For those years in which the DES is not conducted, SSA will need to monitor the potential pool of applicants. One means by which SSA can monitor the size and characteristics of potential beneficiaries is through other ongoing federal data collection efforts. For both the conduct of the DES and monitoring of the pool of potential beneficiaries through the use of various data collection efforts, it is critical to understand the measurement error properties associated with the identification of persons with disabilities as a function of the essential survey conditions under which the data have been and will be collected. The extent to which alternative instruments designed to measure persons with disabilities map to various eligibility criteria under consideration by SSA is also important.
The collection of disability data is an evolving field. Although a large and growing number of scales attempt to measure functional status and work disability, little is known about the measurement error properties of various questions and composite scales. The empirical literature provides clear evidence of variation in the estimates of the number of persons with disabilities in the United States, depending upon the conceptual paradigm of interest, the analytic objectives of the particular measurement process, and the essential survey conditions under which the information is collected (e.g., Haber, 1990; McNeil, 1993; Sampson, 1997). This literature suggests that estimates of the disabled population not only are related to the conceptual framework underlying the measurement construct but are also a function of the essential survey conditions under which the measurement occurred, including the specific questions used to measure disability, the context of the questions, the source of the information (self-versus proxy response), variations in the mode and method of data collection, and the sponsor of the data collection effort. Furthermore, terms such as impairment, disability, functional limitation, and participation are often inconsistently used, resulting in different and conflicting estimates of prevalence. Attempts to measure not only the prevalence but also the severity of an impairment or disability further complicate the measurement process.
Recent shifts in the conceptual paradigm of disability, in which disability is viewed as a dynamic process rather than a static measure and as an interaction between an individual with an impairment and the environment rather than as a characteristic only of the individual, imply that those responsible for the development of disability measures must separate the measurement of the impact of environmental factors in the enablement-disablement process from the measurement of ability. Viewing disability as a dynamic state resulting from an interaction between a person's impairment and a particular environmental context further complicates the assessment of the quality of various survey measures of disability, specifically, the reliability of a measure. As a dynamic characteristic,