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[citation needed] 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.