Signature
YLD_inc = I * DW * L
| Inputs | Definition | Unit |
|---|---|---|
I | Unit: cases | — |
DW | Severity on a scale from 0 (full health) to 1 (equivalent to death) | none |
L | Unit: years | — |
YLD_inc | Healthy years lost over the whole course of the year's new cases | years |
|---|
Function
Years lived with disability counted from incident cases or from prevalent cases of a sequela
Multiplies time spent with a non-fatal health state by its disability weight, counting the time either from the cases that begin in the reference year (all their future health loss, the GBD 1990 incidence perspective) or from everyone living with the state in the reference year (that year's health loss only, the perspective of GBD 2010 onwards and WHO's Global Health Estimates). Prevalence-based YLDs for one sequela are HE-FM-DWT-001 and their comorbidity adjustment HE-FM-DWT-004 on the disability weight page; steady-state prevalence from incidence and duration is HE-FM-XSD-002. Notation follows the Years Lived with Disability article.
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Implementations
Excel
Incidence-based years lived with disability from named cells
With IncCases, DisWeight and DurationYrs named, the formula returns the incidence-based YLDs, held in YLDInc. PrevYears holds the person-years lived with the condition in the same year, for the steady-state check.
=IncCases*DisWeight*DurationYrs
Assumptions
Constant weight over the whole duration of a case
Each case carries the same disability weight for its average duration L, which ends at remission or death; a condition with stages needs one term per sequela.
Future health loss assigned to the year and age of onset
The count credits the reference year with health loss that will be lived over the next L years, so it reflects new cases rather than the health loss lived in that year.
Worked examples
Condition with 100 new cases a year, a weight of 0.20 and a duration of 10 years
This year's 100 new cases give 100 x 0.20 x 10 = 200 YLDs, as in the article's steady state, where the prevalence-based count of 1,000 person-years at 0.20 also gives 200.
I = 100; DW = 0.2; L = 10; YLD_inc = 200
Same condition after a programme halves new cases to 50 a year
The incidence-based count falls at once to 50 x 0.20 x 10 = 100 YLDs, crediting the reference year with all the future health loss of the smaller cohort, as in the article.
I = 50; DW = 0.2; L = 10; YLD_inc = 100
Common errors
Comparing an incidence-based YLD with a figure from GBD 2010 onwards
GBD 2010 and WHO switched to prevalence-based YLDs, so an incidence-based figure counts a different quantity; the two agree only when incidence and duration have been stable for a long time.
Reading the fall in an incidence-based count as a fall in current burden
WHO lists as the first drawback of the incidence approach that it does not reflect the current prevalent burden of a condition whose incidence has fallen substantially; in the article's example the count halves at once while people with earlier onset still live with the condition.
Placing the health loss at the age of onset when ages of experience matter
In an incidence perspective all YLDs for congenital hearing loss fall at age 0, whereas in a prevalence perspective they are spread fairly evenly across age groups, which suits what WHO notes policy-makers are often more interested in, the ages at which health loss is experienced.
Sources
WHO methods for incidence-based YLDs in GBD 1990 and WHO updates
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: the GBD 1990 and subsequent WHO updates used an incidence perspective; the number of incident cases is multiplied by the average duration of the disease and a weight factor from 0 (perfect health) to 1 (dead), YLD = I x DW x L, with L the average duration of the case until remission or death.
WHO on the drawbacks of incidence-based YLDs and the switch to prevalence
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 2.4: the incidence-based approach will not reflect the current prevalent burden of a condition for which incidence has been substantially reduced, needs estimates of both incidence and duration, and assigns all YLDs to the ages at onset; both GBD 2010 and WHO switched to a prevalence-based approach; YLDs for congenital hearing loss fall at age 0 in an incidence perspective.
Murray 1994 on the technical basis of the DALY
Murray CJL. Quantifying the burden of disease: the technical basis for disability-adjusted life years. Bulletin of the World Health Organization. 1994;72(3):429-445 (abstract read). Abstract: sets out the technical basis of the disability-adjusted life year, with a three per cent discount rate and an exponential age-weighting function.
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