Disability-Adjusted Life Year

A Disability-Adjusted Life Year (DALY) measures the loss of healthy life in a population due to illness, disability and premature death. One DALY represents one year of healthy life lost, combining the burden of dying early with the burden of living in less than full health.

How DALYs are calculated

DALYs combine two components: years of life lost (YLL) and years lived with disability (YLD). The relationship is DALY = YLL + YLD.

• YLL measures premature mortality by multiplying deaths at each age by the standard remaining life expectancy at that age, then adding the results.
• YLD measures non-fatal health loss by multiplying the prevalence of each health condition or consequence by its disability weight.
• A disability weight represents the severity of health loss on a scale from 0, equivalent to full health, to 1, equivalent to death; it is not a measure of a person's social worth.

How to interpret the measure

DALYs measure health loss, not healthy years gained. A higher DALY burden therefore indicates a greater loss of healthy life.

• A condition can create a substantial burden through premature deaths, prolonged ill health, or both.
• Unlike a death rate, DALYs capture non-fatal health consequences, allowing conditions that rarely cause death to be included in population health assessment.
• DALYs are not a count of affected people: one person can lose several healthy years, while many people with mild conditions may together account for a smaller loss.

Uses and limitations in health planning

DALYs provide a common unit for comparing the burden of different diseases, injuries and risk factors. This helps policymakers identify major sources of health loss and set priorities for prevention and treatment.

• Comparisons between populations require attention to population size and age structure; total DALYs and age-standardised DALY rates answer different questions.
• Estimates depend on the quality of mortality and morbidity data, modelling assumptions and disability weights.
• DALYs should complement, not replace, assessments of equity and access, because aggregate health loss alone does not show who faces financial hardship or barriers to care.