''disability impact"—the probability or level of disability among people with a given condition.4 Using data from the 1984 Supplement on Aging, Verbrugge and colleagues find a negative relationship between the frequency of a condition and its disability impact. LaPlante (1988) finds a similar relationship in 1983-1985 NHIS data.
The foregoing analyses are based on only the "main" causes of the respondents' activity limitations. Many people who report activity limitations list more than one condition contributing to their limitations. Because there are so many possible combinations of conditions and relatively few people with any particular combination in any survey, analyses of the effect of multiple conditions on disability are limited.
The right half of Table 2-4 shows the prevalence of the 15 most commonly cited conditions responsible for activity limitations, regardless of whether the condition was listed as the main cause. This list is generally similar to the list of main causes. The most notable difference is that hypertension and diabetes move up from ranks 9 and 11 to ranks 4 and 6, respectively, as their prevalence more than doubles. This suggests that a more comprehensive listing of all of the contributing conditions would put more emphasis on chronic diseases than the analysis here of only main causes.
Verbrugge and colleagues have found that as the number of chronic conditions affecting an individual increases, "disability" increases rapidly. This is true when disability is measured in terms of physical or role limitations or by ADL/IADL measures. Only in rare instances, however, is there a synergism between conditions that produce more disability than the two alone would suggest (Verbrugge et al., 1989).
Grouping NHIS data from 1969-1971 and 1979-1981, Rice and LaPlante (1988a) found that the number of chronic conditions reported by those who are limited in their activities increases with age, and that the degree of limitation increases with the number of conditions. The researchers also found that, within every age group and limitation category, the number of conditions has increased over time. It is possible, however, that some of this increase reflects increasing awareness of conditions—perhaps due to improved access to medical care and screening opportunities—rather than a true increase in chronic conditions.
Because they are derived from a single survey, these data on the causes of activity limitation all refer to a cross-section of the population. The "causes"
This is not the probability that a given condition will eventually cause activity limitation. It is simply the ratio of the number of people with limitation caused by a condition to the total number of people with that condition. A cohort rather than a period perspective is needed to calculate the proportion of people with a condition who will ever experience an activity limitation caused by that condition. Incomplete reporting on nondisabling conditions, comorbidities, competing causes, and so on also needs to be taken into account.