3
Models of Disability and Rehabilitation

Models assist understanding by allowing one to examine and think about something that is not the real thing, but that may be similar to the real thing. People use a variety of models to obtain a clearer understanding of a problem or the world around them. Such models include physical models, three-dimensional graphical models, animal models of biological systems, mathematical or ideal models, and computer models. When relationships are highly complex, however, as they are in rehabilitation processes and other areas of human endeavor, it is seldom possible to develop models that are quantitatively predictive. Nevertheless, it is often possible to establish rough relationships between various variables that are observable.

Models based on partial knowledge are often called conceptual models. Conceptual models may help people to think about behaviors of components in complex systems, even though they may not yield quantitative answers. They may allow one to understand general relationships without the necessity for an extensive verbal or written description. In this way they are like an out-of-focus picture that partially reveals relationships. It is common in science and engineering to use models to help develop hypotheses that can be examined experimentally, but even as models assist scientists in moving forward with new understanding, they are abandoned for new versions. Experimental results may suggest that the models must be altered or even abandoned in favor of new models.

The models discussed in this chapter are conceptual in nature.



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--> 3 Models of Disability and Rehabilitation Models assist understanding by allowing one to examine and think about something that is not the real thing, but that may be similar to the real thing. People use a variety of models to obtain a clearer understanding of a problem or the world around them. Such models include physical models, three-dimensional graphical models, animal models of biological systems, mathematical or ideal models, and computer models. When relationships are highly complex, however, as they are in rehabilitation processes and other areas of human endeavor, it is seldom possible to develop models that are quantitatively predictive. Nevertheless, it is often possible to establish rough relationships between various variables that are observable. Models based on partial knowledge are often called conceptual models. Conceptual models may help people to think about behaviors of components in complex systems, even though they may not yield quantitative answers. They may allow one to understand general relationships without the necessity for an extensive verbal or written description. In this way they are like an out-of-focus picture that partially reveals relationships. It is common in science and engineering to use models to help develop hypotheses that can be examined experimentally, but even as models assist scientists in moving forward with new understanding, they are abandoned for new versions. Experimental results may suggest that the models must be altered or even abandoned in favor of new models. The models discussed in this chapter are conceptual in nature.

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--> 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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--> among medical conditions, impairments, functional limitations, and the effects of the interaction of the person with the environment (i.e., handicap, disability, societal limitation) although each uses different nomenclatures for the components. Nagi's model of disability explicitly brought the environment into the conceptualization. His model initiated a search for the factors in family, community, and society that affect disability as an outcome. With respect to disability in the work setting, for example, research has focused on the social and demographic characteristics of the individual and family, the individual's prior occupation and the industry in which the individual was previously employed, the flexibility of the workplace with respect to the physical tasks of work and hours of work, the nature of the local economy, customs and laws governing employment, and the extent of income transfer programs (Yelin, 1992). Although the Nagi model included the environment, it was limited in how it conceived of the environment. In his model, the environment impinges on individuals only when activity limitation interacts with the demands placed on those individuals; the process that gives rise to disability is still inherently a function of the characteristics of medical conditions and attendant impairments. The IOM model (IOM, 1991) was derived directly from Nagi, defining disability as "a function of the interaction of the person with the environment" and beginning to describe certain subsets of environmental factors that could potentially affect the development of and movement within a disabling process. In this model, physical and social environmental risk factors (as well as biological and lifestyle risk factors) were described as independent variables that exist at all stages of the process. These factors affect progression within the model, and their control therefore affects (prevents) disability. The NCMRR model adds emphasis to the importance of environment by adding a category called societal limitations to account for restrictions that society places on individuals and that limit their ability to participate independently in tasks, activities, and roles. The unwillingness of employers to provide accommodations and the lack of ramps that deny access to public buildings to persons with disabilities are given as examples. Building on these models, this committee describes a model where the environment interacts with the individual to determine whether disability will result. Nagi's nomenclature is used in describing the stages of the model and the relational nature of disability, as described in the IOM model, is now enhanced and clarified. In this new model, the environment plays a critical role in determining whether each stage occurs and if transitions between the stages occur.

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--> A New Model for the Enabling-Disabling Process A common understanding of such terms as injury, impairment, handicap, functional limitation, disabling conditions, and disability is essential to building effective, coherent programs in rehabilitation science and engineering. As described above, several frameworks have been advanced to describe disability-related concepts, but none of these has been universally adopted. The lack of a uniformly accepted conceptual foundation is an obstacle to research and to other elements critical to rehabilitation science and engineering. Using the definitions laid out in Chapter 1, this committee presents a new set of models, based primarily on the previous IOM model (1991), designed to enhance the robustness of the previous models with respect to reversing the disabling process, i.e., rehabilitation. This section presents an overview of "the enabling-disabling process," explains its stages, and describes the nature of disability. An Overview of the Enabling-disabling Process An overview of how disabling conditions affect a person's access to the environment is shown in Figure 3-1. Access to the environment, depicted as a square, represents both physical space and social structures (family, community, society). The person's degree of physical access to and social integration into the generalized environment is shown as degree of overlap of the symbolic person and the environmental square. A person who does not manifest disability (a) is fully integrated into society and therefore has full access to both: 1) social opportunities (employment, education, parenthood, leadership roles, etc.) and 2) physical space (i.e. space access equivalent to persons without disabling conditions). A person with potentially disabling conditions1 has increased needs (expressed by the size of the individual) and is dislocated from their prior integration into the environment (b). The rehabilitative process attempts to rectify this displacement, either by restoring function in the individual (c) or by expanding access to the environment (d) (e.g., building ramps). This model does not mean to imply that the two methods (which may be generally characterized as cure and care) are mutually exclusive. Indeed, the most effective rehabilitation programs include both. The model separates the two only to illustrate that disability is the interaction between the potentially disabling 1   It is important to note that a potentially disabling condition becomes an actual disabling condition once the person is dislocated from the environment as a result of that condition.

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--> FIGURE 3-1 Conceptual overview of the enabling-disabling process. The environment, depicted as a square, represents both physical space and social structures (family, community, society). A person who does not manifest a disability (a) is fully integrated into society and "fits within the square." A person with potentially disabling conditions has increased needs (expressed by the size of the individual) and is dislocated from his or her prior integration into the environment (b) that is, "doesn't fit in the square." The enabling (or rehabilitative) process attempts to rectify this displacement, either by restoring function in the individual (c) or by expanding access to the environment (d) (e.g., building ramps).

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--> conditions of an individual and the environment, and therefore strategies that affect the environment or the pertinent potentially disabling conditions both target disability. While this model provides an overview, more detail is provided below. The New IOM Model Looking at the enabling-disabling process with more scrutiny requires greater detail in the model. To this end, this report adopts the IOM model (1991) and makes some modifications designed to both improve the model and to tailor it more towards rehabilitation (see Figure 3-2). The original IOM model was conceived with prevention in mind, and the need for identifying risk factors whose control would facilitate the prevention of disability. The 1991 IOM model (IOM, 1991) established a new conceptual foundation in the field of disability in that it analyzed and described the components of the disabling process in such a way as to allow for the identification of potential points for preventive intervention. Identifying and describing the importance of the different types of risk factors that affect the disabling process as well as the interaction and integral nature of quality of life were fundamental contributions to the emerging field of disability prevention. Over time, however, some shortcomings in the 1991 IOM model have emerged, including the implication that the disabling process is unidirectional, progressing inexorably toward disability without the possibility of reversal. The unidirectionality was implied by the arrows in the model that pointed only to the right, that is, toward the condition of disability. Although this may have been a result of that committee's focus on developing interventions to prevent progression in the disabling process rather than reversal, that is, rehabilitation, it is a shortcoming in the original model that needs correction and clarification, especially in the context of rehabilitation. A second apparent shortcoming in the 1991 IOM model is its limited characterization of the environment and the interaction of the individual with the environment. Although the importance of the environment is discussed in the text in some detail, it is not clearly represented in the model except as a category of risk factors involved in the transition between the various categories of the disabling process. The third apparent shortcoming in the 1991 IOM model that the committee identified as needing improvement is the representation of societal limitation. Some enhancements to the original model address these shortcomings. The new IOM model (Figures 3-2, 3-3, and 3-4) is designed to show disability more clearly as the interaction of the person with the environ-

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--> FIGURE 3-2 Modified IOM model. The Disability in America model (Institute of Medicine, 1991) is revised to include bidirectional arrows and a state of "no disabling condition," and to show transitional factors and quality of life interacting as part of the enabling-disabling process. The state of "disability" does not appear in this model since it is not inherent in the individual but, rather, a function of the interaction of the individual and the environment.

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--> FIGURE 3-3 The person-environment interaction. The enabling-disabling process is depicted as being an active part of the individual person. The physical and social environments are depicted as a three-dimensional mat, with social factors on one side and physical factors on the other. The interaction of the person and the "environmental mat" is depicted as a deflection in the mat. ment and also to show the possibility of movement in the direction of rehabilitation. To accomplish this diagrammatically, the new model is three-dimensional and has the following new features: 1.   The person: Arrows pointing left were added to represent the potential effects of rehabilitation and the "enabling process" (risk factors and enabling factors are now combined into "transitional factors"). In addition, the new model includes the designation "no disabling conditions" to indicate that there is a beginning and an end to the disabling process when a pathology, impairment, functional limitation, or disability does not exist. 2.   The environment: The shaded gray area from the 1991 model becomes "the environment," including the physical, social, and psychological components of the environment, and is represented as a three-dimensional mat that supports and interacts with the person and the disabling process, serving to highlight the importance of the person-environment interaction. 3.   Disability: The box that was labeled "disability" in the 1991 model

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--> FIGURE 3-4 Disability as displacement of the environmental mat. The amount of disability that a person experiences is a function of the interaction between the person and the environment. The amount of displacement in the environmental mat is a function of the strength of the physical and social environments that support an individual and the magnitude of the potentially disabling condition. The amount of displacement represents the amount of disability that is experienced by the individual. has been moved from being a part of the disabling process to being a product of the interaction of the person with the environment. Each of these enhancements is described in greater detail below. Assembling the Model As shown in Figure 3-3, the new model can be shown as having three parts: the person, the environment, and the interaction of the person with the environment (disability). The Person In the new model a new designation was added to indi-

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--> cate people with no disabling conditions. This feature of the model will allow for ''complete" rehabilitation (designating also both the origin of the disabling process and the termination of the enabling process). Transitional Factors In the new model, the committee defines the converse of risk factor as "enabling factor." Risk factors are phenomena that are associated with an increase in the likelihood that an individual will move from left to right in the new model, that is, from no disabling condition toward functional limitation. In contrast, enabling factors are phenomena that are associated with an increase in the likelihood that an individual will move from right to left in the new model, that is, toward less limitation. The general types of enabling factors are the same as the general types of risk factors, that is, environmental (social, psychological, and physical) along with lifestyle and behavioral. So, for example, access to appropriate care and assistive technology would be an enabling factor (social environment), but lack of access would be a risk factor, or a disabling factor; curb cuts and universal design would be enabling factors (physical environment), but a lack of these would be disabling factors; the age of the person is a biological factor that can be either enabling or disabling; and compliance with pharmaceutical prescription regimens would be enabling, whereas drug abuse would be a disabling (behavioral and lifestyle) factor. Thus, since both disabling and enabling factors affect transitions between the stages of the model, the committee groups them together as "transitional factors." The Environment The environment is represented as a flexible three-dimensional mat in the new model. The strength and resilience of this mat are proportional to the quantity and quality of accessible support systems and the existence of various barriers. Stronger mats equate with more supportive environments, for example, access to appropriate health care, the availability of assistive technology and social support networks, and receptive cultures. Weaker mats equate with nonsupportive environments. For example, physical barriers, discrimination, lack of accessible and affordable assistive technology, and lack of appropriate health care result in greater displacement of the mat and, therefore, cause greater disability. Thus, a person with a given level of impairment or functional limitation (i.e., potential disability) will experience greater disability (more displacement of the mat) in a less supportive environment than he or she would experience in a more supportive environment (indicated by a stronger mat and less resulting displacement). The amount of disability is pro-

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--> portional to the amount of displacement in the mat that represents the environment. The environment is represented as having two general categories: the social-psychological and the physical. Examples of the types of things that might be included in each category include: Psychological and Social Environments Discrimination Access to health and medical care Appropriate care Access to technology Culture Employment Family Economy Community organizations Access to social services Traits and personality factors Attitudes and emotional states Access to fitness and health-promoting activities Education Spirituality Independence Physical Environments Architecture Transportation Climate Appropriate technology Geography Time Each of the items listed in the social and physical environments could be thought of as layers in the mat; for example, access to assistive technology would be a layer in the environment mat, and so if an individual had good access to assistive technology, a strong layer for assistive technology would be added to the mat. If there were no access to assistive technology, then this layer would be missing from the mat, thus weakening the overall support and increasing the resultant disability. In keeping with this model, it is important to note that the environment interacts at all points in the process (e.g., the environmental risk factors described in Disability in America [Institute of Medicine, 1991]). Reflecting the increasing focus on the interaction of the individual

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--> and the environment, recent research on disability and rehabilitation has described the constituent parts of the environment in as much detail as Nagi's model gave to the individual pathway (Fawcett et al., 1994; Fougeyrollas and Gray, 1996; Fougeyrollas, 1997; Law et al., 1996). These researchers see the environment of the person with a disabling condition as including elements that are proximate, such as the immediate home and work environments (termed the microsystem of the individual), and distal, such as the community in which the individual lives (termed the mesosystem) and the society, economy, and, perhaps above all, the culture in which the local community is embedded (termed the macrosyste). However, these researchers do not include psychological or intrapersonal factors as part of the microsystem, an omission that the enhancements of the model described in this chapter are meant to rectify. In this chapter, psychological factors such as one's thoughts, beliefs, or expectancies are included in the intrapersonal environment. Although the person with a disabling condition experiences the microsystem tangibly every day, the extent to which a particular condition is expressed as a disabling condition may be determined as much at the macro- or mesosystem level as by the nature of the local environment. For example, research on disability in the work setting indicates that the economic status of the overall labor market has a far greater impact on the employment status of people with disabling conditions than the willingness of individual employers to provide accommodations or the extent of the physical or mental impairment for that matter, even though both accommodations and extent of impairment do have some effect (Yelin, 1992). Similarly, the overall culture frequently determines whether a limitation will be considered disabling. In the broader U.S. culture, for example, a severe limitation in hearing is considered a disability. In a society in which the culture supports the use of sign language, a hearing loss may not be limiting (Groce, 1985). The relative importance of the different elements of the environment may differ among kinds of activities. The extent of family help and the nature of the landscape and the built environment—microsystem and mesosystem characteristics, respectively—may affect an individual's ability to get around the community more than the overall culture (macrosystem characteristics). Even in that example, however, the importance of the overall culture becomes clear, because in some societies the provision of such services flows quite naturally from a communitarian ethos, whereas in others it is left to individuals and families to fend for themselves. Disability The definition of disability has not changed, but its representation in the model has. In the new model, disability is a dependent variable whose value is determined by the relationship between two other

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--> variables: the person and the environment. Since disability is not a part of the person, but rather is a function of the interaction of the person with the environment, the box that represented disability in the 1991 IOM model has been removed from the person component in the new model (see Figure 3-4). Disability is now represented as the quantity of deflection in the mat that represents the environment. Thus, in the new model disability is a relational outcome. Although many parts of the process are not well understood at present, the areas in which knowledge is strong and those in which it is weak can be specified in the new model. Theoretical Quantification and Mathematical Model of Disability As a relational concept, disability lends itself to mathematical modeling. At present this can be done only on a conceptual basis, since quantification of the variables is not yet reliable and reproducible. Nonetheless, a mathematical model is useful in further clarifying the relationship that exists between the person and the environment and how they interact to create disability. Beginning with the variables of pathology (P), impairment (I), and functional limitation (FL), the first assumption is that the sum of these variables represents a quantity known as an individual's potential disability, or PD: P + I + FL = PD. Potential disability is referred to as "potential" because it is only the interaction of potential disability with the environment that creates true disability. In addition, some (or all) of these variables may be equal to zero. If all are equal to zero, then there is no potential disability. Environment (E) is the denominator in the mathematical model, because it is the influence of the environment that creates disability from any given potential. The environment is the variable factor against which the other factors are measured and that determines the existence of disability. Without environmental factors there would be no disability. Thus, disability (D) can be represented mathematically as the relationship of potential disability (PD, or P + I + FL) to environment (E): (P + I + FL)/E = D or PD/E = D. Take the following example. If the range of PD (i.e., the sum of P + I + FL) is 0 to 10 (where 10 is the maximum potential for experiencing disability) and the range of E is 0.1 to 10 (where 1.0 is a neutral environment, 10 is a very accessible and supportive environment, and 0.1 is a very inaccessible and nonsupportive environment), then the resultant quantity of D would range from 0 to 100 (where 100 is maximum disability). So, for example, if PD were 5 and the environment were a very "good"

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--> TABLE 3-1 Estimating the Effects of the Environment on Disability Functional Capacity (qualitative) Potential Disability (qualitative) Potential Disability (range: 1-10) Environmental Support (range: 0.1-10) Disability Score (range: 0.1-100) High Low 1 10 (good) 0.1     1 1 (neutral) 1     1 0.5 (bad) 2     1 0.1 (worst) 10 Moderate Moderate 5 10 (good) 0.5     5 1 (neutral) 5     5 0.5 (bad) 10     5 0.1 (worst) 50 Low High 10 10 (good) 1     10 1 (neutral) 10     10 0.5 (bad) 20     10 0.1 (worst) 100 one (e.g., E = 10), then the level of disability would be 0.5. However, with the same PD (i.e., PD = 5) but a less supportive environment (e.g., E = 1), then the level of disability would increase from 0.5 to 5. (When E is equal to 1, the effect of the environment is neutral and PD is equal to D.) Mathematically, to allow the environment to exert a truly negative effect in the model, the range of quantities for the denominator (i.e., the environment) should be less than 1.0 but greater than zero; thus, the range would be from 0.1 to 10. Using the example above, if E is equal to 0.5 instead of 1, then the level of disability would increase from 5 to 10. If the environment was at its worst (i.e., E = 0.1), then the amount of disability would increase to 50 in this example. In summary, maintaining the level of potential disability constant (PD = 5) and varying the quality of the environment (E is from 0.1 to 10) results in the moderate disability scores listed in Table 3-1 for the example cited above. Conceptual Matrix for Rehabilitation Research The conceptual models presented here could form the basis for a new science—rehabilitation science and engineering—that embodies the underlying, rudimentary paradigm implicit in each. The committee believes that there is a legitimate and distinct field of study that justifies the term "rehabilitation science." Rehabilitation science emphasizes function, focusing on the processes by which disability develops and the factors in-

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--> fluencing these processes. The vision of rehabilitation science is that better understanding of the causes and factors contributing to disability will lead to better treatments and technology for those with disabling conditions. To help define the parameters of rehabilitation science and engineering, the committee developed the conceptual matrix presented in Table 3-2. The matrix is useful as a tool for identifying the focus of rehabilitation science and engineering, defining which research activities address rehabilitation, and providing new possibilities for future research. Table 3-2 demonstrates that rehabilitation science and engineering targets the enabling-disabling process, which runs across the stages of disability, and addresses the disabling conditions and the environment. The cells of this matrix (i.e., the different letters, Al through M4) match disability-related variables in the rows to performance measures in the columns. A single cell (e.g., Al or C3) in the matrix constitutes the minimum requirement for research classified as associated with the new academic field of rehabilitation science and engineering. If a row variable is studied across several performance states, the multi-cell begins to represent the enabling-disabling process. Every research study in rehabilitation science and engineering must have some measure of disability-related function (columns) and some measure of a disability-related variable, either of the person or of the environment (rows). It is not sufficient to study a classification of illness or disease without a measure of performance. This requirement defines the subset of pathology and pathophysiology research in medicine that overlaps with rehabilitation science and engineering. It also serves as a guide for defining the rehabilitation science and engineering relevant research in basic biological, physical, behavioral, and social sciences and in the health professional and engineering disciplines. The highest priority in rehabilitation science and engineering is for studies yielding causal explanations of disabling and rehabilitative processes. The matrix also reveals the unique nature of rehabilitation science and engineering. First, rehabilitation science emphasizes function. Second, rehabilitation science focuses on factors that lead to transitions between pathology, impairments, functional limitation, and disability. Third, rehabilitation science examines physical, behavioral, environmental, and societal factors that influence these transitions. Although rehabilitation science is multidisciplinary and utilizes methods from many fields including medicine, biomedical engineering, material sciences, sociology, architecture, and even economics, it is distinct from other scientific disciplines due to its emphasis on function, factors, and interventions that disable or enable people. Rehabilitation science and engineering combines knowledge from these otherwise distinct disciplines to create a knowledge structure for the understanding of performance deficits un-

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--> TABLE 3-2 Conceptual Parameters of Rehabilitation Science and Engineering   Disability-Related Performance Measures (rehabilitation realms)     Pathology Impairment Functional Limitation Disability   Enabling Processes     <—————————————————————   Personal and Environmental Variables Disabling Processes     —————————————————————>   Personal (e.g., biology, behavior, resources)         Natural processes (growth, maturation, development, aging, repair) A1 A2 A3 A4 Disease (manifestations and progression) B1 B2 B3 B4 Behavioral lifestyle C1 C2 C3 C4 Experiential (e.g., perceptions, autonomy, well-being, quality of life) D1 D2 D3 D4 Resources (e.g., physical, monetary, skills) E1 E2 E3 E4 External environment Natural (e.g., climate, terrain) F1 F2 F3 F4 Culture G1 G2 G3 G4 Engineered (e.g., assistive technology, architecture, transportation) Hl H2 H3 H4 Therapeutic modalities I1 I2 I3 I4 Health care delivery systems J1 J2 J3 J4 Social institutions (e.g., family, religion) K1 K2 K3 K4 Macroeconomic indicators (e.g., gross national product, unemployment rates) L1 L2 L3 L4 Policy and law M1 M2 M3 M4 Resources and opportunities N1 N2 N3 N4

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--> derlying disability, the processes in which disabling conditions develop, progress, and reverse, and the factors that mediate the disabling and enabling processes. Table 3-2 finally creates possibilities for new research. The matrix identifies combinations of variables which can be researched, but are not likely studied in the separate, existing basic science, health professional, and engineering disciplines. The variables in the rows (i.e., person and environmental variables) may be investigated as dependent or independent variables with the performance realms (columns) as the respective independent or dependent variable counterpart. Switching the independent variable from column to row will allow elucidation of the impact of disabling or rehabilitative processes on the person or society, on the one hand, or the impact of person or environmental variables (as positive or negative effectors) on enabling and disabling processes. The new knowledge of rehabilitation science and engineering will stimulate insights into rehabilitative processes and innovations into rehabilitation therapeutics. The matrix in Table 3-2 can also be used to evaluate current data collection efforts with respect to the prevalence of and impact of disability. Currently, federal surveys routinely collect information on the extent of pathology, impairments, and on limitations in major classifications of activities. The National Health Interview Survey collects some information on behavioral factors and in the use of health care. In addition, some episodic surveys such as the Longitudinal Study on Aging and Health and Retirement Survey collect some information on accommodations provided by government, family, and employers. However, no ongoing surveys routinely collect information on the experiential and resource domains within personal factors and none collect information on such external environmental factors as the natural and engineered environment, access to rehabilitation specific therapies and health care, the availability of social supports, and the experience of the laws and regulations governing disability policy. In short, information on disability is almost exclusively within the personal domains and even so, tends to focus on definitions of disability that accord with much more primitive models of disability. Summary and Conclusions The prevailing wisdom about the causes of disability has shifted dramatically in the span of the last several decades. From the deterministic position of the American Medical Association Committee on Impairments of the 1950s, in which pathology was viewed as equivalent to disability, to the probabilistic one in more recent years of Nagi, IOM, and National Center for Medical and Rehabilitation Research (NCMRR) that, although

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--> still focusing on the characteristics of the individual, viewed the relationship among pathology, impairment, functional limitation, and disability as contingent, to the view espoused here, that the environment and characteristics of the individual conjointly determine disability, the underlying notion of why someone is unable to function in the major activities of life has been transformed. The notion that disability arises from the interaction of a person's physical condition and the person's environment has several implications. These implications affect research, professional training, and policy. Although there is not space here to discuss these implications in depth, their outlines will be sketched briefly in the hopes that others will investigate them further. One implication of the notion that environments can be enabling is that disability does not always have to be viewed through a "deficit" model. Rather, it should be recognized that in an environment that is strongly enabling, a person with a substantial physical impairment can live a life that is indistinguishable in important ways from that of a similar person without such an impairment. That is, the person with the impairment can hold a good job, be married and have a family, engage in non-work-related activities, enjoy social relationships, and be part of a community. Not much research has been done from this perspective. Some exceptions are the works of Gray (1996), and White et al. (1995). The committee strongly recommends that more research be done which emphasizes the effects of enabling environments on the lives of people with impairments and functional limitations. If disability is a function of the environment, it is not a stable attribute across situations. Rather, it will depend on the particular environment or the particular situation. For example, it is possible that a person with a certain type of impairment might be more disabled in his or her work environment than in the family environment. This would be true, for example, of a deaf person working with hearing coworkers who did not know how to sign, who worked for a company that provided interpreters only for large or important meetings, but whose spouse and children all signed fluently. Or an impairment might affect a person's ability to use public transportation, but not the ability to work once arriving in the workplace. For research, this means that one measurement of impairment or functional limitation cannot be developed to indicate degree of disability in all situations. One measurement that attempts to indicate how disabled a person is would be misleading. If the measurement were limited to a specific, clearly defined situation, such as a workplace, it might be possible to include enough environmental indicators that a valid and reliable measurement might be developed; however, this measurement

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--> is unlikely to apply across all situations. This will, for the short run, make the tasks facing researchers much harder. This conception of disability also carries with it implications for professional training. If the goal of professional training in rehabilitation is to impart the skills needed to reduce disability, training programs will need to teach many forms of environmental modification. Training may need to be broader than it has been, and it will need to include more professional fields and skills (see Chapter 9 for further discussion of this issue). Finally, even though disability policy is not the focus of this report, it is worth noting that the policy implications arising from this definition of the concept are enormous. Disability determination will not be able to be a single event. Rather, a determination of disability status, for example, for the purpose of receiving Social Security Disability Income benefits, will have to be tied to a specific time, place, set of skills, and type of job. It cannot be permanent, for not only will changes in the person's own health or educational status change it, but so will changes in aspects of the work environment. Additionally, any single disability determination might not be acceptable across programs, since different programs apply to different environments. Recommendations Recommendation 3.1 Researchers and educators should adopt the model for rehabilitation and the enabling-disabling process presented here. Programs supporting rehabilitation-related research should adopt its terminology and use it as an organizing tool. Recommendation 3.2 Based on the model of the enabling-disabling process described in this report, methods for quantifying disability should be developed that are sensitive to the characteristics of both the person and the environment. Such a metric would facilitate additional research into the factors that affect transitions between disability and other states of the enabling-disabling process, and the development of effective preventive and rehabilitative intervention strategies.