Disability-Adjusted Life Year
Original Editor - Wendy Walker
Introduction & definition
DALY = Disability Adjusted Life Year = the sum of years of potential life lost due to premature death and the years of productive life lost due to disability compared to a standardised life expectancy.
DALYs are used to measure the combined quantity and quality of life of a population.
The WHO website summarises the DALY thus:
"One DALY can be thought of as one lost year of 'healthy' life. The sum of these DALYs across the population, or the burden of disease, can be thought of as a measurement of the gap between the current health status and an ideal health situation where the entire population lives to an advanced age, free of disease and disability."
i.e. one DALY is equivalent to the loss of a healthy life year.
Calculation of DALY
This measurement is a combination of two measurements (shown below):
DALY = YLL + YLD
i.e. Disability Adjusted Life Year [DALY] is equal to the sum of the Years of Life Lost [YLL] due to premature mortality in the population and the Years Lost due to Disability [YLD} for people living with the health condition.
Years of Life Lost (YLL)
This consists of the number of deaths multiplied by the standard life expectancy at the age at which death occurs.
The formula for this is: YLL = N x L
N = number of deaths and L = standard life expectancy at age of death (in years)
Years Living with a Disability (YLD)
There are two methods of calculating the YLD.
1. The number of cases, in a particular time period, multiplied by the average duration of the disease combined with a weight factor which represents the severity of the disease on a scale from 0 (perfect health) to 1 (dead).
The formula is: YLD = I x DW x L
- I = number of incident cases
- DW = disability weight
- L - average duration of he case until remission or death (in years)
2. Known as the Prevalence YLD:
The formula is: YLD = P = DW
- P = number of prevalent cases
- DW = disability weight
Function and Uses of DALYs
DALYs are used to indicate the impact of disease on a population.
They measure the difference between the current situation and an ideal situation where everyone lives to the standard life expectancy and is in perfect health. For this calculation, the standard life expectancy is set at 80 years for men and 82.5 years for women.
DALYs for a number of diseases can be added together to indicate the total burden of disease in a country.
DALYs are a summary metric of population health. DALYs represent a health gap; they measure the state of a population's health compared to a normative goal. The goal is for individuals to live the standard life expectancy in full health.
Advantages of DALYs
1. The biggest advantage is that morbidity (YLD) and mortality (YLL) effects are combined in one measure.
2. DALY allow the comparison between different health hazards.
3. The DALY measure offers the ability to assess the impact of prevention strategies
Limitations of DALYs
DALYs are based on baseline measurements from the wealthy countries so it is likely that the differential found between developing and developed populations means that DALYs may not reflect the differences in resources.
The disability weights used for YLD are based on subjective measures, whereby the applied technique and the used panel (e.g. experts, patients or lay-people) have a strong influence on the obtained disability weights. There is an on-going debate over their validity. The DALY measure is not widely recognized outside the health sector.
The biggest challenges of the DALY measure consist in:
- Getting estimates of the total number of infections in the population due to a particular pathogen.
- When using the incidence and pathogen-based approach, defining the outcome tree with all relevant health outcomes for a particular pathogen.
- Getting appropriated transmission probabilities for the different health outcomes represented in the outcome tree.
- Murray, C. (1994). “Quantifying the burden of disease: the technical basis for disability-adjusted life years.” Bulletin of the World Health Organization 72(3): 429-445
- Cassini A, Colzani E, Pini A, Mangen MJ, Plass D, McDonald SA et al. Impact of infectious diseases on population health using incidence-based disability-adjusted life years (DALYs): results from the Burden of Communicable Diseases in Europe study, European Union and European Economic Area countries, 2009 to 2013. Euro Surveill. 2018;23(16):17-00454.
- CJL Murray et al. Disability-adjusted life years (DALYs) for 291 diseases and injuries in 21 regions, 1990–2010: a systematic analysis for the Global Burden of Disease Study 2010. The Lancet, 15 Dec 2012, Vol 380, No 9859, 02197-2223
- Anand, S., & Hanson, K. (1997). “Disability Adjusted Life Years: A Critical Perspective.” Journal of Health Economics 16:685-702
- Parks, R, The Rise, Critique and Persistence of the DALY in GLobal Health. Journal of Global Health, 10 Aug 2014