Over the years various investigators have attempted to overcome these limitations by developing new metrics that factor in disability or quality of life along with mortality. One of the most recent, and still controversial, measures is the Disability-Adjusted Life Year, or DALY.
Disability-adjusted life year is a measure of overall disease burden, expressed as the number of years lost due to ill-health, disability or early death.
The DALY was first conceptualized by Murray and Lopez in work carried out with the World Health Organization and the World Bank known as the global burden of disease study, which was published in 1990. It is now a key metric employed by the United Nations World Health Organization in such publications as its Global Burden of Disease.
Today the DALY is becoming an increasingly common term in the field of public health and health impact assessment. It extends the concept of potential years of life lost due to premature death to include equivalent years of healthy life lost by virtue of being in states of poor health or disability - mortality and morbidity are combined into a single, common metric.
Looking at the burden of disease via DALYs can reveal surprising things about a population's health. For example, the 1990 WHO report indicated that 5 of the 10 leading causes of disability were psychiatric conditions. Psychiatric and neurologic conditions account for 28% of all years lived with disability, but only 1.4% of all deaths and 1.1% of years of life lost. Thus, psychiatric disorders, while traditionally not regarded as a major epidemiological problem, are shown by consideration of disability years to have a huge impact on populations.
Disability Adjusted Life Year Formula:
One DALY can be thought of as one lost year of "healthy" life. The sum of these DALYs across the population, or the burden of disease, can be thought of as a measurement of the gap between current health status and an ideal health situation where the entire population lives to an advanced age, free of disease and disability.
Traditionally, health liabilities were expressed using one measure: (expected or average number of) Years of Life Lost (YLL). Japanese life expectancy statistics are used as the standard for measuring premature death, as the Japanese have the longest life expectancies.
This measure does not take the impact of disability into account, which can be expressed by: Years Lived with Disability (YLD). DALYs are calculated by taking the sum of these two components.
DALY = YLL + YLD
The DALY relies on acceptance that the most appropriate measure of the effects of chronic illness is time, both time lost due to premature death and time spent disabled by disease.
One DALY, therefore, is equal to one year of healthy life lost.
Some studies use DALYs calculated to place greater value on a year lived as a young adult. This formula produces average values around age 10 and age 55, a peak around age 25, and lowest values among very young children and very old people.
A crucial distinction among DALY studies is the use of "social weighting", in which the value of each year of life depends on age. Commonly, years lived as a young adult are valued more highly than years spent as a young child or older adult. This weighting system reflects society's interest in productivity and receiving a return on its investment in educating children. Society has invested relatively little in very young children, and it has already received a substantial return on its investment in older people. Young adults, however, have received the maximum amount of investment and, at the beginning of their working life, have had the least opportunity for society to benefit from its investment.
The global burden of disease (GBD) 2001-2002 study counted life years equally, but the GBD 1990 and GBD 2004 studies used the following formula:
W = 0.1658 Y e-0.04 Y
Where Y is the age at which the year is lived and W is the value assigned to it relative to an average value of 1.
In these studies future years were also discounted at a 3% rate, so that a weighted year of life saved next year is worth 97% of a year of life saved this year.
The effects of the interplay between life expectancy and years lost, discounting, and social weighting are complex, depending on the severity and duration of illness. For example, the parameters used in the GBD 1990 study generally give greater weight to deaths at any year prior to age 39 than afterward, with the death of a newborn weighted at 33 DALYs and the death of someone aged 5 to 20 weighted at approximately 36 DALYs.