Research Council, 1990). As defined and described throughout this report, much of "disability" is a social issue, going beyond biological or functional limitations and relating to people's ability to perform their expected social roles. This chapter, however, attempts to avoid the debate about how disability should be defined and measured, and simply reports on the data that are available.

The National Health Interview Survey (NHIS) is a population survey that has been conducted continuously by the National Center for Health Statistics for almost 30 years (National Center for Health Statistics, 1989a). The NHIS data, particularly the data on "activity limitation," provide a reasonably consistent national picture over a long period of time, and hence are used as the framework of the synthesis in this chapter. These data are supplemented, where appropriate, by data from other surveys described below.

The National Health Interview Survey (NHIS)

The NHIS is designed to collect representative data on the civilian noninstitutionalized population living in the United States. Among those excluded from the scope of the NHIS are residents of nursing homes, members of the armed forces, prisoners, and U.S. citizens living abroad. In 1988, the survey reached a sample of 122,310 persons in 47,485 households. The use of households to locate survey respondents means that the NHIS tends to underrepresent that portion of the population, the homeless, for example, that do not live in households. To the extent possible, adults are interviewed directly. Proxy respondents provide information for all children in the household and for those adults who cannot be interviewed in person (National Center for Health Statistics, 1989a).

The NHIS data on activity limitation are obtained through questions that establish whether an "impairment or health problem" prevents or limits activities and whether that impairment or health problem is chronic. Respondents are further classified according to the degree of activity limitation: (1) limited, but not in "major activity" (the least severely limited category); (2) limited in amount or kind of "major activity"; or (3) unable to carry out "major activity'' (the most severely limited category). "Major activity" is defined as the predominant social role expected of a person of a given age. According to the current definition, the major activities are "playing" for children under age 5, "attending school" for children ages 5-17, "working or keeping house" for adults ages 18-69, and "living independently" for adults age 70 and over. As discussed below in the section on trends, however, these definitions have changed over time (National Center for Health Statistics, 1989a).

A chronic condition is one that has existed for at least three months or

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