Signature
DALY = YLL + YLD
| Inputs | Definition | Unit |
|---|---|---|
YLL | Years of life lost to deaths from the cause, deaths times the standard remaining life expectancy at the age of death (HE-FM-YLL-001) | years |
YLD | Years lived with disability from the cause, summed over its sequelae (HE-FM-DWT-001 or HE-FM-YLD-001) | years |
DALY | Disability-adjusted life years from the cause for the given sex, age group and year | years of healthy life lost |
|---|
Function
Disability-adjusted life years for a cause or a scenario from years of life lost and years lived with disability
Maps the deaths and the non-fatal health loss from a cause, or in one scenario of an economic evaluation, to disability-adjusted life years (DALYs) by adding the years of life lost (YLL) to the years lived with disability (YLD), each calculated by its own formula. YLLs are deaths times a standard loss function of the age at death (HE-FN-YLL-001, HE-FM-YLL-001, with the discounted form HE-FM-YLL-003 and the age-group total HE-CF-YLL-001), and YLDs are prevalent cases times a disability weight (HE-FM-DWT-001) or incident cases times the weight and the duration (HE-FN-YLD-001, HE-FM-YLD-001, discounted in HE-FM-YLD-002). This package adds the sum itself, the DALYs of a cohort in one scenario, DALYs averted and cost per DALY averted between two scenarios, the continuously discounted cohort form and the DALY rate per 100,000 population. DALYs averted by a change in disability weights alone, with mortality unchanged, are HE-FM-DWT-005, and the general incremental ratio is HE-FM-ICER-001. Notation follows the Disability-Adjusted Life Year article.
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Implementations
Excel
DALYs for a cause from named YLL and YLD cells
With the cause's years of life lost in YLLCause and its years lived with disability in YLDCause, the first formula returns the DALYs, held in DALYCause, and the second the share of the total that comes from deaths, held in ShareYLL.
=YLLCause+YLDCause; =YLLCause/DALYCause
Assumptions
YLL and YLD for one cause counted for the same population and year
Both parts refer to the same cause, sex, age group, location and year, as in WHO's formula DALY(c,s,a,t) = YLL(c,s,a,t) + YLD(c,s,a,t). Adding YLLs for one year to YLDs for another, or for a different age group, gives a figure that belongs to no population.
One set of value choices for both parts of a DALY
The two parts use the same conventions. From GBD 2010 onwards, and in WHO estimates after its decision in late 2012, both are undiscounted and not age-weighted, YLDs are counted by prevalence with an adjustment for comorbidity, and YLLs use a normative standard life table. The original GBD 1990 DALY counted YLDs by incidence and applied a 3% discount rate and age weights to both parts.
Worked examples
Global DALYs in 2021 summed from GBD 2021 years of life lost and years lived with disability
GBD 2021 reported 1.98 billion YLLs and 907 million YLDs worldwide in 2021, entered here in millions. Their sum is 2,887 million against the reported total of 2.88 billion, a gap that comes from rounding in the published figures, and YLLs supply about 69% of the total (68.7% as reported).
YLL = 1980; YLD = 907; DALY = 2887
Untreated cohort in the article's DALY example summed from its two parts
In the article's illustrative cohort without treatment, 20 deaths at age 40 give 900 YLLs and 180 survivors living 10 years at a weight of 0.3 give 540 YLDs, a total of 1,440 DALYs, of which deaths supply 62.5%.
YLL = 900; YLD = 540; DALY = 1440
Purely fatal cause in a DALY sum as a limiting case
For a cause with no period of illness counted, YLD is zero and the DALY equals the YLL, here the 900 years from the 20 deaths in the article's example. This limiting case checks that a blank YLD cell is read as zero.
YLL = 900; YLD = 0; DALY = 900
Common errors
Adding discounted years of life lost to undiscounted years lived with disability
Discounting one part and not the other mixes conventions. In the article's untreated cohort, YLLs discounted continuously at 3% (493.84) added to undiscounted YLDs (540) give 1,033.84 DALYs, which matches neither the undiscounted 1,440 nor the discounted 960.37 (computed here for illustration).
Summing cause-specific DALYs built from YLDs not adjusted for comorbidity
YLDs computed separately for each cause count the health loss of people with several conditions more than once, so a sum over causes overstates total DALYs. GBD 2010 and later rounds adjust YLDs for comorbidity before adding them to YLLs (HE-FM-DWT-004 on the disability weight page).
Sources
WHO formula for DALYs as the sum of YLL and YLD by cause, sex, age and year
World Health Organization. WHO methods and data sources for global burden of disease estimates 2000-2019. Global Health Estimates Technical Paper WHO/DDI/DNA/GHE/2020.3. Geneva, WHO, 2020 (full text read). Section 2, which calculates DALYs for a cause as the sum of the YLLs from that cause and the YLDs for people living in states of less than good health resulting from it, DALY(c,s,a,t) = YLL(c,s,a,t) + YLD(c,s,a,t) for cause c, age a, sex s and year t. Section 2.1 lists the GBD 2010 changes, including adjustment for comorbidity in YLDs and no discounting or unequal age weights, and section 2.3 the original 3% rate and age weights.
GBD 2021 DALYs calculated by summing YLDs and YLLs
GBD 2021 Diseases and Injuries Collaborators. Global incidence, prevalence, years lived with disability (YLDs), disability-adjusted life-years (DALYs), and healthy life expectancy (HALE) for 371 diseases and injuries in 204 countries and territories and 811 subnational locations, 1990-2021, a systematic analysis for the Global Burden of Disease Study 2021. The Lancet. 2024;403(10440) 2133-2161 (full text read). Methods, where YLLs are deaths multiplied by the standard life expectancy at the age of death and DALYs are calculated by summing YLDs and YLLs. Results, with 907 million YLDs (31.3%) and 1.98 billion YLLs (68.7%) of the 2.88 billion DALYs in 2021.
Canonical Identity
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