The findings in this report are subject to at least five limitations. First, despite complex statistical adjustment procedures used to address nonresponse over time, these procedures may not have completely eliminated bias that resulted from nonresponse errors, especially in subgroup analyses. Second, this report excluded persons in institutions, in the military, and aged <18 years. Third, persons with multiple disabilities may attribute the main disability to the one most disabling at the time of the interview, which may result in inconsistent survey responses. Fourth, because of questionnaire design, the main associated health condition was determined for most but not all adults with disability; 2.9 million (6.4%) persons whose only disabilities were difficulty with vision, hearing, or speech, who had selected impairments, used assistive aids, or received federal disability benefits were not asked about a main condition. Finally, the definition of disability used did not assess environmental and social barriers, discrimination as the result of disability, and effects on the workforce. These issues are addressed in the International Classification of Functioning, Disability, and Health (ICIDH-2), a unified and standard framework that describes the dimensions of disability.9ICIDH-2 complements the International Classification of Diseases by organizing information around three dimensions: body level (body systems and structure), person and society level (activities and participation), and the environment. Because of the dynamic quality of disability, a limitation in one dimension does not predict a limitation in another.