DALY Calculation Method

The methodology used to compute disability-adjusted life years, combining years of life lost to premature death with years lived with disability, weighted by severity.

Source: Murray 1994

A disability-adjusted life year combines two components. Years of life lost measure premature death, calculated as the difference between the age at death and a standard life expectancy at that age, taken from a reference table applied to every population so the loss does not depend on where a person lived. Years lived with disability measure time in impaired health, calculated as the duration of a condition multiplied by a weight reflecting its severity, on a scale from zero for full health to one for a state equivalent to death. The two are summed, so one disability-adjusted life year is one year of healthy life lost.

Source: Murray 1994

The weights are elicited from surveys presenting lay descriptions of health states and asking respondents to make comparative judgements between them, most commonly by choosing which of two described people is healthier. Responses across large samples are then combined into a single scale using statistical methods that convert the paired comparisons into cardinal weights. Earlier rounds used expert panels and person trade-off exercises with small groups of health professionals, which attracted criticism for reflecting professional rather than public views. Current weights are derived from population surveys conducted across many countries, and the finding that judgements were broadly consistent across settings is what supports applying a single set of weights globally rather than country-specific ones.

Source: Salomon et al. 2012

Two adjustments were applied in the original formulation and have since been dropped. Age weighting counted years lived at different ages differently, rising through young adulthood and declining later, on the argument that people in those years support dependants. Discounting reduced the weight of years lost further in the future, applying the logic used for costs. Both were removed in the 2010 revision of the global burden estimates, on the grounds that they introduced value judgements into a measure meant to describe burden rather than prescribe priorities. Removing them raised the recorded burden of conditions causing death in childhood, so estimates published either side of the change are not comparable.

Source: Salomon et al. 2012

A disability-adjusted life year measures health lost while a quality-adjusted life year measures health gained, so the two run in opposite directions and their weighting scales are inverted, with zero representing full health for one and death for the other. Disability weights are derived from comparative judgements about described states, whereas quality weights are derived from trade-offs involving time or risk, which embed a willingness to give something up. Disability-adjusted life years use a standard life expectancy applied uniformly, while quality-adjusted life years use the actual survival modelled for the population concerned. The measures were built for different purposes, burden description against decision analysis, and converting between them is not straightforward despite their apparent symmetry.

Source: Murray 1994

The disability weights rest on judgements by respondents who have not experienced most of the states described, and people living with a condition frequently rate it less severely than the general population does. Using a single standard life expectancy means a death at a given age counts identically everywhere, which is defended as equity and criticised as ignoring what was realistically achievable in the setting. The measure aggregates across very different kinds of loss, so a large number of mild impairments can register the same burden as a smaller number of severe ones. And because the method has been revised substantially, series spanning a revision cannot be interpreted as showing change in health.

Source: Anand & Hanson 1997