HC Statistics

Why have disability rates decreased?

Although the evidence was mixed for the 1980s and it is difficult to pinpoint when in the 1990s the decline began, during the mid- and late 1990s, the panel found consistent declines on the order of  0-2.5% per year for two commonly used measures in the disability literature: difficulty with daily activities and help with daily activities.

From the quotation above, we see that disability trends have been decreasing over time.  The question is, why is this?  Was the decrease in disability cause by decreasing rates of chronic disease or were decreasing disability rates caused by decreased disability rates among those with chronic diseases?  This is the question Aranovich, Bhattacharya, Garber and MaCurdy attempt to answer in their recent working paper “Coping with Chronic Disease?

Let us assume an individual has a disability in year t when Dt=1 and the person has a chronic disease when Ct=1. According to the equation below

  • P[Dt]=P[Dt|Ct]*P[Ct=1] + disability from non-chronically ill population.

The authors calculate the probability an individual has a chronic disease P[Ct], using data from the National Health Interview Survey (NHIS).  The authors calculate [Dt|Ct] using Bayes rule as follows:

  • P[Dt|Ct]=P[Ct|Dt]*P[Dt]/P[Ct]

The numerator is calculated using the National Long Term Care Survey (NLTCS) and the denominator we already calculated from the NHIS.  Now we can decompose the changes in disability rates into the following:

  • ΔP[Dt] = ΔP[Dt|Ct=1]P[Ct=1] + P[Dt|Ct]ΔP[Ct=1] + change attributable to non-chronically ill pop.

This says that the change in disability is a mixture of the change in the prevalence of chronic disease and the change in th probability of being disabled given that you have a chronic disease.


Disability can be defined in one of two ways: IADLs or ADLs.  “IADLs include everyday behaviors such as grocery shopping, managing money, and preparing meals and are considered a measure of moderate disability.  The ADL measure, which encompasses more basic, mechanically-oriented  activities, including dressing, eating, and bathing, is considered a gauge of more severe forms of functional impairment.”

  • Overall Disability.  Between 1982 and 1999, the authors found a decrease in IADL disability of 45% whereas ADL disability decrease by 9%.  
  • Chronic Disease Rates P[Ct].  In general, the authors found increases in the age-adjusted prevalence of chronic diseases.   The prevalence of being overweight increased by 10.4 percentage points, arthritis rates increased 3.0 and diabetes prevalence increased by 1.1.  There were small increases in the prevalence of stroke, chronic obstructive pulmonary disease (COPD).  On the other hand the prevalence hypertension and heart disease decreased by 2.6 percentage points and 3.3 percentage point respectively.
  • Probability of Disability for those with a Chronic disease: P[Dt|Ct]. Between 1982 and 1999, people with arthritis, hypertension, COPD, overweight and heart diseases all experienced about a 50% decline in IADL disability.  Disability of those with diabetes decreased by 25%.  “Among the seven conditions evaluated, only overweight was associated with a statistically significant decline (p<.05) in ADL disability between 1982 and 1999, a decrease of about 20%.”  However, there were also smaller, non-statistically significant declines in ADL disability among those with heart disease, COPD and arthritis.


Overall, we see a trend of decreasing disability rates and increasing rates of chronic illness.  This means that disability levels have decreased for those who have chronic disease.  It does not seem to be the case that preventive care is decreasing the level of chronic illness.  It could be the case, however, that as more people live longer, observing more chronic illness is an improvement from the counterfactual of death rather than a counterfactual of no disability.  It is also important to note that IADL disability decreased more than ADL disability.  This could be explained by environmental factors.  For instance, “[i]nternet shopping, amplifying devices for phones, and street ramps” all would help to decrease IADL levels, but would have little effect on ADL levels.


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