Burgess COMMENTARY

Peter Burgess

Metrics: Disability-Adjusted Life Year (DALY) Quantifying the Burden of Disease from mortality and morbidity Definition 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 current health status and an ideal health situation where the entire population lives to an advanced age, free of disease and disability.

DALYs for a disease or health condition are calculated as 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 or its consequences:

Calculation

DALY = YLL + YLD

The 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

Because YLL measure the incident stream of lost years of life due to deaths, an incidence perspective has also been taken for the calculation of YLD in the original Global Burden of Disease Study for year 1990 and in subsequent WHO updates for years 2000 to 2004.

To estimate YLD for a particular cause in a particular time period, the number of incident cases in that period is multiplied by the average duration of the disease and a weight factor that reflects the severity of the disease on a scale from 0 (perfect health) to 1 (dead). The basic formula for YLD is the following (again, without applying social preferences):

YLD = I x DW x L

where:
I = number of incident cases
DW = disability weight
L = average duration of the case until remission or death (years)

Prevalence YLD

The recent GBD 2010 study published by IHME in December 2012 used an updated life expectancy standard for the calculation of YLL and based the YLD calculation on prevalence rather than incidence:

YLD = P x DW

where:
P = number of prevalent cases
DW = disability weight

Social value weights (age-weighting and discounting)

The original Global Burden of Disease Study and WHO updates for years 2000-2004 also applied several social value weights in the calculation of DALYs for diseases and injuries. Apart from the disability weights, these also included time discounting and age weights. For more information on these, select the link on the right-hand side of this page.

Disability weights, discounting and age weighting of DALYs Egalitarian principles are explicitly built into the Disability-Adjusted Live Year (DALY) metric, and the global burden of disease studies apply these to all regions of the world. The studies use the same 'ideal' life expectancy for all population subgroups and exclude 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, they use the same 'disability weight' for everyone living a year in a specified health state. DISABILITY WEIGHTS A disability weight is a weight factor that reflects the severity of the disease on a scale from 0 (perfect health) to 1 (equivalent to death). Years Lost due to Disability (YLD) are calculated by multiplying the incident cases by duration and disability weight for the condition. The disability weights used for the GBD 2004 are listed here. AGE WEIGHTING AND DISCOUNTING 3% discounting and non-uniform age weighting was used in the original GBD 1990 study. These adjustments result in less weight given to years lived at young and older ages. The GBD 2001-2 study used 3% discounting but uniform age weighting. The GBD 2004 update used the original 3% discounting and non-uniform age weighting. GBD 2004 estimates of DALYs are available for standard DALYs (3% discounting and age weights), no frills DALYs (no discounting, no age weights) and discounted DALYs 3% discounting, no age weights) here. Because prevalence is approximately incidence x duration, prevalence YLD for a condition (across all ages) is approximately the same as the no frills incidence YLD. However, when discounting or age weighting are applied, the prevalence YLD for a condition may be quite different in magnitude to the incidence-based YLD.