listed are actually chronic conditions that may have had very different underlying causes earlier in life. Blindness, for instance, could be a congenital condition or the result of a developmental problem; it could also be due to an injury or a disease such as diabetes. The NHIS data cannot distinguish between these very different possibilities. Furthermore, disability is a dynamic process in which illness and injuries that occur in one life stage have serious implications for the quality of life in later stages. The life table calculations below are a first step toward putting disability in a life course perspective.

LIFE TABLE PERSPECTIVE

Just as one can calculate the average length of life in a population exposed to certain mortality rates, one can also calculate the total number of years a member of this population would spend with various levels of activity limitation. Ideally, this calculation would be carried out using age-specific transition rates among the various categories of activity limitation, just as age-specific mortality rates are used to describe transitions from life to death in ordinary life tables (Rogers, R.G., et al., 1989). Because the necessary transition data are not available for activity limitation, the committee has adopted a hybrid approach (Sullivan, 1971; McKinlay et al., 1989). First, standard life table methods were used to calculate the number of years of life experienced by a cohort in various age groups according to the 1987 U.S. life table (National Center for Health Statistics, 1990c). Second, within each age group for which LaPlante (1988) has tabulated the NHIS activity limitation data, a calculation was made of the number of years lived with various kinds of activity limitations. Based on these results, the committee then calculated the life expectancy without disability and in the various activity-limited states.5 Because the NHIS figures refer to only the noninstitutionalized population, the committee's calculations underestimate time with activity limitations.

Given current age patterns of activity limitation, an average of 12.8 years out of the current life expectancy at birth of 75.0 years would be spent with some degree of activity limitation. As shown in Figure 2-9, an average of 6.9 of the 16.9 years remaining at age 65 would be spent with some activity limitation. At age 75, the remaining 10.7 years would be expected to include 4.6 with some activity limitation. This analysis suggests that, if current patterns of mortality and activity limitation continue, the 3.8 million children born in 1987 can expect to experience a collective total of 49

5  

Specifically, the committee partitioned nLx values from the 1987 life table according to the proportions of people with different kinds of activity limitations in the relevant age groups from the NHIS. Just as life expectancy at age x (ex) would be calculated by summing the nLx for age x and above, the life expectancy with a particular kind of activity limitation was calculated by summing the appropriate components of nLx.



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