LaPlante (1989) and others (Haber, 1967; Nagi, 1976) define disability as a limitation, caused by one or more chronic physical or mental health conditions, in performing activities that people of a particular age are generally expected to be able to perform. For children, these activities might include playing and going to school. For adults, these activities might include employment or running a household.
LaPlante (1989) notes that chronic conditions are the antecedents of disability, yet there are many chronic conditions that differ in terms of functional limitations, prognosis, care management, and rehabilitative potential--factors that relate to or describe the nature of a disability. Because of differences in age, other physical and socioeconomic factors, motivation levels, environmental factors, treatment resources, duration of the underlying illness, and care management patterns, two people with the same chronic condition of equal severity may not have equal levels of disability. For example, severe arthritis may be devastating for a surgeon, but not as much of a limitation for a writer who has a computer and voice recognition software. Thus, the mere existence of a chronic condition is not sufficient to specify the degree of disability that people face in their own situations.
Because of the range of factors that influence perceptions of disability, disability research frequently relies on surveys. Some examples, cited by Irvin et al. (1994), include the Current Population Survey, the Survey of Income and Program Participation, and the National Medical Expenditure Survey. LaPlante (1989) describes the usefulness of the National Health Interview Survey for disability studies. As Irvin et al. (1994) note, disability research relies on self-reports of health-related limitations in activities and work based on these surveys, and on participation in government programs for disabled people that require certain medical or functional status criteria to be met (see, for example, Burkhauser,Haveman, and Wolfe, 1992; Haveman, Wolfe, Buron, and Hill, 1992; Wolfe and Haveman, 1990; Haveman and Wolfe, 1983; and Colvez and Blanchet, 1981). With the exception of the National Health Interview Survey, the surveys that support this body of research often do not provide information about the pathology or impairment underlying the self-report, or they do not link functional limitations to specific conditions (Irvin, et al., 1994).
While survey data are necessary for assessing the degree of disability that individuals face, one formidable obstacle to the use of survey data should be noted--survey respondents may not always respond accurately or in the expected manner. This may be due to confusing wording of survey questions, lack of complete response categories, a respondent's unwillingness to divulge information, or other problems. In their own study and in many other studies which they reviewed, Fowles et al. (1998) note that the accuracy of self-reported data on the existence of chronic diseases varies substantially with the type of disease. Although the Fowleset al. study does not deal specifically with disabling conditions, the tendency of respondents to underreport chronic conditions is likely to translate into an underestimate of disabling conditions as well.
Using medical records data as a gold standard, Fowles et al. found that low sensitivity (i.e., underidentification of those with a disease) was particularly problematic, usually more often for self-reported data than for data coming from insurance claims. However, some conditions are not well defined in that article (e.g., alcohol abuse, digestive problems, joint problems) and most conditions had a low frequency of occurrence.
Irvin et al. (1994) note that research incorporating both self-reported limitations and specific medical conditions has broadened operational definitions of disability. These broader definitions attempt to capture the multi-dimensional nature of disability. Nagi(1969, 1979) argues that this multi-dimensional nature should include the elements of "pathology" (the presence of a physical or mental malfunction), "impairment" (physiological, anatomical, or mental losses or abnormalities that limit a person's capacities and level of functioning), and "disability" (inability or limitations in performing roles and tasks that are socially expected). Haveman, de Jong, and Wolfe (1991) consider the multi-dimensional nature of disability using a disability index.
Irvin, et al. (1994) used data from the National Medical Expenditure Survey to identify four different groups of disabled people. These groups were not mutually exclusive. The first group included those with chronic conditions and impairments reported during a medical visit. A second group included those who self-reported one or more limitations in activities of daily living (e.g., bathing, toileting) and those who reported that their health limited moderate or vigorous physical activity. A third group of disabled people was defined as those respondents who reported that their health limited the kind or amount of work they could perform. A fourth group included respondents who reported that their health prevented them from working at all. When all four groups were combined, Irvin et al. (1994) found that they represented approximately 40 percent of the 1987 National Medical Expenditure Survey sample of working age individuals. Irvin et al. (1994) note that definitions of disability range from broad constructs reflecting social and economic conditions and self-perceptions to tightly defined physical and mental conditions entailing the application of strict medical criteria. However, all of these measures are based on subjective measures of pathologies or impairments.
LaPlante (1989) cites research showing that about one-half of the U.S. population aged 18-64 has at least one chronic physical or mental health condition, but only about 15 percent of that group have one or more activity limitations. Thus, LaPlante notes: It is a simple fact that most people with chronic conditions are not limited in activity, nor do most conditions cause disability (page 20).