Signature
DW = 1 - N_ref / N_eq
| Inputs | Definition | Unit |
|---|---|---|
N_ref | Number of healthy people whose one-year life extension is the reference, such as 1,000 | people |
N_eq | Number of people with the condition whose one-year life extension the respondent judges equal to the reference, at least N_ref | people |
DW | Weight implied by the respondent's equivalence | none |
|---|
Function
Disability weight scaling of time lived with a non-fatal health state
Maps time lived in a non-fatal health state to healthy time lost by multiplying it by the state's disability weight, on a scale where 0 is equivalent to full health and 1 to death. The products give years lived with disability (YLD), which add to years of life lost to give DALYs. When people have several conditions, weights are combined multiplicatively and the combined loss is shared among the conditions. The notation follows the Disability Weight article.
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Implementations
Excel
First person trade-off weight from named numbers
With the reference group in RefPeople and the equivalent group in EquivPeople, the formula returns the weight, held in PTOOneWeight.
=1-RefPeople/EquivPeople
Assumptions
Person trade-off judgements of equal claim on resources
The respondent compares life extensions for two groups and states the size at which they have an equal claim; the weight follows from the ratio alone.
Worked examples
GBD 1990 example of 8,000 people with a severe disability
If 1,000 healthy people have an equal claim to 8,000 people with a severe disability, the weight is 1 minus 1,000 / 8,000, or 0.875.
N_ref = 1000; N_eq = 8000; DW = 0.875
First person trade-off disability weight from an equivalence of 1,250 people
If 1,250 people with a condition are judged equal to 1,000 healthy people, the weight is 0.2 (computed here for illustration).
N_ref = 1000; N_eq = 1250; DW = 0.2
Common errors
Forcing agreement between the two person trade-off questions
GBD 1990 panellists revised PTO1 and PTO2 answers until they matched; Arnesen and Nord argue that forced consistency produced artefacts rather than real preferences.
Sources
First person trade-off weight as one minus 1,000 divided by 8,000
Arnesen T, Nord E. The value of DALY life: problems with ethics and validity of disability adjusted life years. BMJ. 1999;319(7222):1423-1425. doi:10.1136/bmj.319.7222.1423. Section on the person trade-off: quoting the GBD summary, if 1,000 healthy people would have an equal claim on resources as 8,000 people with some severe disability, the weight is 1 minus 1,000 divided by 8,000, or 0.875; PTO1 presupposes that the lifetime of disabled people is devalued.
Canonical Identity
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