DALY

Disability adjusted life years (DALY)
DALYs for a disease are the sum of the years of life lost due to premature mortality (YLL) in the population and the years lost due to disability (YLD) for incident cases of the health condition. The DALY is a health gap measure that extends the concept of potential years of life lost due to premature death (PYLL) to include equivalent years of 'healthy' life lost in states of less than full health, broadly termed disability. One DALY represents the loss of one year of equivalent full health.
DEFINITION
The Disability Adjusted Life Year or DALY is a health gap measure that extends the concept of potential years of life lost due to premature death (PYLL) to include equivalent years of ‘healthy’ life lost by virtue of being in states of poor health or disability (1). The DALY combines in one measure the time lived with disability and the time lost due to premature mortality. One DALY can be thought of as one lost year of ‘healthy’ life and the burden of disease as a measurement of the gap between current health status and an ideal situation where everyone lives into old age free of disease and disability.
YEARS OF LIFE LOST PLUS YEARS LOST TO DISABILITY
DALYs for a disease or health condition are calculated as the sum of the years of life lost due to premature mortality (YLL) in the population and the years lost due to disability (YLD) for incident cases of the health condition:
DALY= YLL + YLD
The years of life lost (YLL) basically correspond to the number of deaths multiplied by the standard life expectancy at the age at which death occurs. The basic formula for YLL (without yet including other social preferences discussed below), is the following for a given cause, age and sex:

YLL= N X L
Where:
• N = number of deaths
• L = standard life expectancy at age of death in years
YLD=1 X DW X L
Where:
• I = number of incident cases
• DW = disability weight
• L = average duration of the case until remission or death (years)

DISABILITY WEIGHTS
The disability weights used in the GBD 2000 are listed in detail elsewhere (2). Egalitarian principles were explicitly built into the DALY, and the Global Burden of Disease Study used the same values for all regions of the world (3) . It used the same life expectancy ‘ideal’ standard for all population subgroups and it excluded all non-health characteristics (such as race, socioeconomic status or occupation) apart from age and sex from consideration in calculating lost years of healthy life. Most importantly, it used the same ‘disability weight’ for everyone living a year in a specified health state. Additionally, 3% time discounting and non-uniform age weights which give less weight to years lived at young and older ages were used in calculating DALYs for the original Global Burden of Disease study (3). These value choices have continued to be used for the Global Burden of Disease 2000 study results reported in recent World Health Reports (4).
With non-uniform age weights and 3% discounting, a death in infancy corresponds to 33 DALYs, and deaths at ages 5 to 20 to around 36 DALYs. Thus a disease burden of 3,300 DALYs in a population would be the equivalent of 100 infant deaths or to approximately 5,500 persons aged 50 years living one year with blindness (disability weight 0.6).