to the United States, these surveys have grown increasingly sophisticated, and by the 1950s Katz and Lawton had developed short sets of survey questions based on behavioral theories of human function. These indexes are known as the Activities of Daily Living (ADLs), which measure abilities in six functions (bathing, dressing, toileting, transfer, continence, and feeding), and the Instrumental Activities of Daily Living (IADLs), which are concerned with more complex tasks (e.g., shopping, cooking, housekeeping, laundry, use of transportation, managing money, managing medications, use of telephones).9 Another measure, limitations in major activities, asks respondents about their ability to work, attend school, or perform housework.5 (A list of many of the major surveys, including those with questions on disability, can be found in the National Research Council publication, The Aging Population in the Twenty-first Century: Statistics for Health Policy .15) Each of these measures has been widely used—or retooled for use—in surveys and functional assessment inventories.

Despite their widespread use, however, each of the classification systems that have come into use in this country suffers from limitations of one kind or another. From a research perspective, the use of self-reported measures raises questions concerning the standardization of the participant's answers. Disability measures have also been problematic as public policy-making tools. The eligibility criteria of the nation's social security insurance programs have been criticized for relying on the narrowly defined criteria of the disease model to determine disability.7,10 In health policy, the ADLs and IADLs have similar constraints. Although not dependent on the disease model, they do not have a systematic means of defining psychological difficulties, and they do not provide insight into certain social contributions to disability, such as discrimination. The systems measuring limitations in major activities, on the other hand, may indicate the presence of some social contributions to disability but do not provide sufficient information to inform health interventions. These limitations have been recognized and other forms of classification, including attempts to combine these measures, have been attempted, but there has been no consensus on a system that could provide a sufficiently broad understanding of disability. These and other specific measures provide insights into the way disabilities affect important parts of most people's lives, but their scope is too narrow. Moreover, health care measures are now called on to assess the quality of life, but without a fuller perspective on the effects of disability, such classification and measurement systems do not have a convincing claim to make such assessments.

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