Disability-adjusted life year

A disability-adjusted life year (DALY) is a measure of overall disease burden, representing a year lost due to ill-health, disability, or early death. It was developed in the 1990s as a way of comparing the overall health and life expectancy of different countries.

The concept has become more common in the field of public health and health impact assessment (HIA). It combines both potential years of life lost due to premature death (mortality) and to poor health or disability (morbidity) into a single metric.

Both DALY and QALY (quality-adjusted life year) are examples of health-adjusted life years (HALY). QALY weights the life expectancy according to the quality of life, which can be reduced by disease or recovered by health care. QALYs are commonly used to assess the cost-effectiveness of medical or public health interventions and to guide decisions aimed at improving social welfare.

The disability-adjusted life year (DALY) is a societal measure of the disease or disability burden in populations. It is calculated by combining measures of life expectancy as well as the adjusted quality of life during a burdensome disease or disability for a population.

Traditionally, health liabilities were expressed using one measure, the years of life lost (YLL) due to dying early. A medical condition that did not result in dying younger than expected was not counted. The burden of living with a disease or disability is measured by the years lost due to disability (YLD) component, sometimes also known as years lost due to disease or years lived with disability/disease.

DALYs are calculated by taking the sum of these two components:

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.

How much a medical condition affects a person is called the disability weight (DW). This 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 results from the specific condition.