The disability-adjusted life year (DALY) is a measure of overall disease burden. Originally developed by the World Health Organization, it 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.
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.
A table comparing the PTSD DALY rates for the world's 25 most populous countries may be found in Epidemiology section of the Wikipedia Posttraumatic stress disorder (PTSD) article. Comparison of this rate across these countries makes clear that the region of the world most impacted by PTSD is Asia.
The DALY was first conceptualized by Murray and Lopez in work carried out with the World Health Organisation and the World Bank known as the global burden of disease study, which was published in 1996. It is now a key metric employed by the United Nations World Health Organization in such publications as its Global Burden of Disease