Over the last several decades, several overarching perspectives have emerged that guide efforts to enumerate the extent of disability in the United States population. Over time these perspectives have evolved from a strictly medical model to one that recognizes the social and environmental context of disability (Fujiura and Rutkowski-Kmitta 2001). The traditional medical model emphasizes the individuals medical condition or organ impairment underlying the disability. In contrast, Nagis functional limitation model (Nagi 1965, 1991; Verbrugge and Jette 1994) and the original International Classification of Impairments, Disabilities, and Handicaps (ICIDH) make explicit the social context of disability.
Nagis functional limitation model emphasizes four stages, first, pathology, or compromised organ function due to chronic or acute conditions or injury; second, impairment, or the loss of system function; third, functional limitations, defined as limitations in physical or mental actions due to the loss in system function; and finally, disability, or the inability to carry out socially defined roles or activities. In this paradigm, disability exists if the functional loss is sufficient to restrict an individual from performance of a socially defined role. Similarly, the ICIDH depicted a four-stage sequence of disorder, impairment, disability, and handicap. In this approach, disability is defined as a limitation in activity, whereas handicap refers to a disadvantage relative to others that is caused by an activity limitation.
The concepts embedded in these models are evident in many of the disability measures that are in national surveys today. For example, concepts embedded in the classic medical model are evident in questions in the NHIS and the Survey of Income and Program Participation, which allow adults with limitations in activities to be classified according to conditions. Similarly, the most recent Census asks about the presence of a condition that substantially limits one or more basic physical activities such as walking, climbing stairs, reaching, lifting, or carrying. Nagis influence is readily seen in the adoption of functional limitation items on most national surveys.
More recently, the International Classification of Functioning, Disability and Health (ICF) has linked health conditions to participation in society through the influence of body functions and structure, activities, the environment, and personal factors (World Health Organization 1999). Unlike earlier frameworks, the ICF explicitly links health dimensions to participation in society and makes explicit contextual factors--the individuals health condition, the environment, and other personal factors--that may influence and interact with the process by which body functions and structures relate to participation.
Several recent measurement-development projects build on these conceptual frameworks. Based on Nagis disablement model, for example, Jette and colleagues (2002a, 2002b) developed questionnaire items that assess late-life function and disability. Similarly, Gill et al. (1998) and Jette (1994) have demonstrated the distinct but complementary nature of measures of difficulty (difficulty with task) and dependence (need for help with task). Freedman and Agree (2005) highlight the role of assistive technology in the disablement process by making explicit the concepts of the environment and accommodations (i.e., the use of help, assistive technology, or changes in behavior). And, drawing upon the conceptual elements of the ICF model of disability, The Washington Group has proposed the development of internationally comparable census measures of disability to assess equalization of opportunity (Madans et al. 2004).