Such models must constantly be changed as new knowledge is gained if they are to adequately represent processes or systems that are in flux. Rehabilitation science and engineering, at its current stage of development, does not have a comprehensive paradigm or a universally accepted theoretical model. It is an emerging field of study, and as such, is still evolving. This chapter presents a brief look at the history of models of disability, which is useful in understanding the current status and direction of disability and rehabilitation research, and then presents a model of disability that builds upon and elaborates previous models, as well as adding several new elements. It presents this model verbally, schematically, and mathematically. Finally, it introduces a matrix that defines rehabilitation research.

Evolution of models of Disability

The prevailing wisdom about the causes of disability has changed in the last several decades. In the 1950s, impairment of a given severity was viewed as sufficient to result in disability in all circumstances; in contrast, the absence of impairment of that severity was thought to be sufficient grounds to deny disability benefits. Thus, the American Medical Association's Committee on Medical Rating of Physical Impairments stated that "competent evaluation of permanent impairment requires adequate and complete medical examination, accurate objective measure of function, and avoidance of subjective impressions and nonmedical factors such as the patient's age, sex and occupation" (American Medical Association, Committee on Medical Rating of Physical Impairment, 1958).

By the mid-1970s, Nagi (1976) outlined a process by which a pathology (e.g., arthritis) gave rise to an impairment (e.g., a limited range of motion in a joint), which may then result in a limitation in function (e.g., an inability to type), which, finally, may result in a disability (inability to work as a secretary). While outlining a process that would seem to move inexorably from pathology to loss of a job, Nagi noted that correlations among impairments, functional limitations, and work loss were poor, and he speculated that the extent to which the environment accommodated limitations largely determined whether disability would result from the onset of a medical condition. In the interim, at least three others have developed models or modifications: the WHO (International Classification of Impairments, Disabilities, and Handicaps, 1980), the IOM (Disability in America, 1991), and the NCMRR (1993). All of these models attempt to facilitate and improve understanding by describing the concepts and relationships

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