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.
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.