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. :
DALY = YLL + YLD 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. Examples of the disability weight are shown on the right. Some of these are "short term", and the long-term weights may be different. The most noticeable change between the 2004 and 2010 figures for disability weights above are for blindness as it was considered the weights are a measure of health rather than well-being (or welfare) and a blind person is not considered to be ill. "In the terminology, the term disability is used broadly to refer to departures from optimal health in any of the important domains of health." At the population level, the disease burden as measured by DALYs is calculated by adding YLL to YLD. YLL uses the life expectancy at the time of death. YLD is determined by the number of years disabled weighted by level of disability caused by a disability or disease using the formula: :
YLD = I × DW × 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. Number of years lost due to premature death is calculated by :
YLL = N × L where N = number of deaths due to condition, L = standard life expectancy at age of death. Historically
Japanese
life expectancy statistics have been used as the standard for measuring premature death, as the Japanese have the longest life expectancies. Other approaches have since emerged, include using national life tables for YLL calculations, or using the reference life table derived by the GBD study.
Age weighting The
World Health Organization (WHO) used age weighting and time discounting at 3 percent in DALYs prior to 2010 but discontinued using them starting in 2010. 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 criticism 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 70 for ten years. This age-weighting function is by no means a universal methodology in studies, but is common when using DALYs. Cost-effectiveness studies using , for example, do not discount time at different ages differently. W=0.1658 Y e^{-0.04Y} where Y is the age at which the year is lived and W is the value assigned to it relative to an average value of 1. In these studies, future years were also
discounted at a 3% rate to account for future health care losses.
Time discounting, which is separate from the age-weighting 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. As a result of numerous discussions, by 2010 the
World Health Organization had abandoned the ideas of age weighting and time discounting. They had also substituted the idea of
prevalence for
incidence (when a condition started) because this is what surveys measure. == Economic applications ==