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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 type of health adjusted life year (HALY) that attempts to quantify the burden of disease or disability in populations. They are similar to quality of life adjust life year (QALY) measures, but rather than attach health related quality of life (HRQL) estimates to health states, DALYs assign HRQLs to specific diseases and disabilities. The methodology was originally developed by the World Bank, but has since been greatly modified and is not an economic measure. However, unique among disease measures, HALYs, including DALYs and QALYs, are especially useful in guiding the allocation of health resources as they provide a common denominator, allowing for the expression of utility in terms of DALYs/dollar, or QALY/dollar. For example, in Gambia, provision of the pneumococcal conjugate vaccine costs $670 per DALY saved.
A crucial distinction among DALY studies is the use of "social weighting", in which the value of each year of life depends on age. There are two components to this differential accounting of time, age weighting and time discounting. Age weighting is based on the theory of human capital. Commonly, years lived as a young adult are valued more highly than years spent as a young child or older adult, as these are years of peak productivity. Age weighting receives considerable flak from those who criticize it for valuing young adults at the expense of children and the old. Some criticize, while others rationalize, this as reflecting society's interest in productivity and receiving a return on its investment in raising children. This age weighting system means that somebody disabled at 30 years of age, for ten years, would be measured as having a higher loss of DALYs (a greater burden of disease), than somebody disabled by the same disease or injury at the age of sixty for 15 years. This age-weight function is by no means a universal methodology in HALY studies, but is common when using DALYs. Cost effectiveness studies using QALYs, for example, do not discount time at different ages differently. It is important to note that this age weighting function applies to the calculation of DALYs lost due to disability. Years lost to premature death are determined from the age at death and life expectancy.
 where is the age at which the year is lived and is the value assigned to it relative to an average value of 1. This age weighting function is not the same as the disability weight (DW) which is determined by disease or disability and does not vary with age. Tables have been created of thousands of diseases and disabilities, ranging from Alzheimer's disease to loss of finger, with the disability weight meant to indicate the level of disability that result from the specific condition.
At the population level, the burden of disease as measured by DALYs is calculated by DALY = YLL + YLD where YLL is years of life lost, and YLD is years lived with disability. In turn, population YLD is determined by the number of years disabled weighed by level of disability caused by a disability or disease using the formula YLD = I x DW x L. In this formula I = number of incident cases in the population, DW = disability weight of specific condition, and L = average duration of the case until remission or death (years). There is also a prevalence (as opposed to incidence) based calculation for YLD. Premature death is calculate by YLL = N x L, where N = number of deaths due to condition, L = standard life expectancy at age of death (expectancy - age at death).
In these studies future years were also discounted at a 3% rate to account for future health care losses. Time discounting, which is distinct from the age weight function, describes preferences in time as used in economic models.
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.
Although some have criticized DALYs as essentially an economic measure of human productive capacity for the affected individual, this is not so. DALYs do have an age weight function that has been rationalized based on the economic productivity of persons at that age, but health related quality of life measures are used to determine the disability weights, which range from 0 to 1 for all disease. These weights are based not on a person's ability to work, but rather on the effects of the disability on the person's life in general. This is why mental illness is one of the leading diseases as measured by global burden of disease studies, with depression accounting for 51.84 million DALYs. Perinatal conditions, which affect infants with a very low age-weight function, are the leading cause of lost DALYs at 90.48 million. Measles is fifteenth at 23.11 million.