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The disability-adjusted life year (DALY) is a measure of overall disease burden, expressed as the number of years lost due to ill-health, disability or early death.
Originally developed by Harvard University for the World Bank in 1990, the World Health Organization subsequently adopted the method in 1996 as part of the Ad hoc Committee on Health Research "Investing in Health Research & Development" report. The DALY is becoming increasingly common in the field of public health and health impact assessment (HIA). 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." In so doing, mortality and morbidity are combined into a single, common metric.
Traditionally, health liabilities were expressed using one measure: (expected or average number of) 'Years of Life Lost' (YLL). 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. In a formula:
The DALY relies on an 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. Japanese life expectancy statistics are used as the standard for measuring premature death, as the Japanese have the longest life expectancies.
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.
The disability-adjusted life year is a metric that expresses only the economic value of a given life year of a given individual to members of society other than that individual; its users do not make any claim (and indeed often take pains from the outset to disclaim) that it expresses the full social value of that year. The difference arises from the facts that a) the metric excludes the net utility that an individual would experience had s/he lived for that year, b) many aspects of an individual's contribution to society are difficult to quantify, and c) many aspects of an individual's value, both as a member of society and in his/her own right, do not admit of quantification and indeed are rooted in the fact that the individual has a unique dignity that cannot be measured by "trade-offs" against those of other 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.
where is the age at which the year is lived and 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 5–20 weighted at approximately 36 DALYs.
Cancer (25.1/1,000), cardiovascular (23.8/1,000), mental problems (17.6/1,000), neurological (15.7/1,000), chronic respiratory (9.4/1,000) and diabetes (7.2/1,000) are the main causes of good years of expected life lost to disease or premature death. Despite this, Australia has one of the longest life expectancies in the world.
These illustrate the problematic diseases and outbreaks occurring in 2013 in Zimbabwe, shown to be most highly impacted by Health Disability: Typhoid, Anthrax, Malaria, Common Diarrhoea, Dysentery 
Posttraumatic stress disorder (PTSD) DALY estimates from 2004 for the world's 25 most populous countries give Asian/Pacific countries and the United States as the places where PTSD impact is most concentrated (as shown here).
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 measure employed by the United Nations World Health Organization in such publications as its Global Burden of Disease
DALYs are essentially an economic measure of human productive capacity for the affected individual, and consequently do not capture other aspects of diseases, such as emotional effects on the individual, friends and families. For instance mentally ill people are sometimes able to continue working normally whilst suffering great distress.