Disability-adjusted life years
Disability-adjusted life years (DALY) is a measure for the overall "burden of disease." Originally developed by the World Health Organization, it is becoming increasingly common in the field of public health and health impact assessment (HIA). It is designed to quantify the impact of premature death and disability on a population by combining them into a single, comparable measure. In so doing, mortality and morbidity are combined into a single, common metric. The DALY was first conceptualized by Murray and Lopez in work carried out with the World Health Organisation and the World Bank, published in 1996.
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: DALY = YLL + YLD.
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.