DALY Computation

Years of life lost (YLL) measure the mortality burden: for each death, the years between the age at death and a standard reference life expectancy, summed over deaths. Years lived with disability (YLD) measure the non-fatal burden: the time people spend living with a condition, weighted by its severity through a disability weight, and computed from incidence times duration or from prevalence. YLL captures dying too soon; YLD captures living in less than full health. A DALY is simply their sum, putting both forms of health loss in the same unit of healthy years lost.

A disability weight is a number between zero and one that represents the severity of a health state, where zero is full health and one is a state regarded as equivalent to death. A weight of 0.3, for instance, means a year lived in that state counts as losing thirty percent of a healthy year. Murray established the technical basis for these weights; early versions came from expert valuation exercises, while later Global Burden of Disease rounds derived them from large general-population surveys asking people to compare health states. They are the device that converts time lived in illness into comparable lost healthy time.

The original DALY gave more value to years lived in young adulthood (age weighting) and valued near-term health gains above distant ones (discounting), both explicit social-value choices Murray built into the formula. They were always controversial because they imply, for example, that saving a young adult counts for more than saving an infant or an elderly person. To make the metric more transparent and equitable and to avoid embedding contested value judgments, later Global Burden of Disease studies set both the age-weighting modulation and the discount rate to zero, so a year of healthy life now counts equally regardless of age or timing, though analysts may reintroduce them in sensitivity analyses.