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The term disability is defined in various ways. In some contexts it is defined in terms of health conditions; in other contexts it is defined in terms of functional limitations; and in still other settings it is defined in terms of activity and role limitations. These varying definitions of disability have in some cases been codified into law, into standardized data collection instruments, and into the practice framework of professionals and organizations that serve people with disabilities. One consequence of the different ways in which disability is defined is that before the characteristics and needs of people with disabilities can be discussed, the parameters of the disability definition being used must be addressed. Whatever the specific components of the definition, there does appear to be some consensus that a person with a disability is someone who experiences limitations in function as a consequence of a permanent physical or mental impairment or a chronic health or mental health condition in interaction with the person's environment. The health condition or impairment may be one that is visible, or it may be invisible. Onset may occur at any age or it may be present at birth. Finally, the severity of disability may vary, even among people with the same condition or impairment, such that some individuals may find it difficult to participate in many life activities, while others experience the effects of disability in a single area.

Among the many definitions of disability used by professionals, government programs, service agencies, and individuals with disabilities, there are three that are most dominant. The first definition involves the extent of limitation in the Activities of Daily Living (ADL) and Instrumental Activities of Daily Living (IADL). The second construct for defining disability is based upon a model developed by Saad Nagi that defines disability in terms of the interaction of environment, functional limitation, and impairment.1,2 The third definition is embodied in the International Classification of Functioning, Disability, and Health (known as ICF) of the World Health Organization (WHO). A fourth measure, used in epidemiological contexts, does not define disability, but it tries to account for the severity of disability by measuring what is referred to as “disability adjusted life years” (DALY).


The ADL scale measures disability in terms of limitations in the Activities of Daily Living. This scale was developed by Katz and coworkers in the 1950s, and has been used extensively by researchers studying the elderly.3 The ADL scale asks about the need for assistance in the activities of eating, bathing, dressing, transfer, and toileting. A related measure, developed by Lawton and Brody in 1969, is the IADL scale.3 The items in this scale ask about the need for assistance in such activities as everyday household chores, managing finances, shopping, and getting around outside one's home. The scales are now used to define levels ...

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