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Disability-Adjusted Life Years (DALYs)

The disability-adjusted life year (DALY) measure combines nonfatal outcomes and mortality in a single summary measure of population health. One DALY represents 1 lost year of healthy life. The basic philosophy associated with the estimation of DALYs is (a) use the best available data, (b) make corrections for major known biases in available measurements to improve cross-population comparability, and (c) use internal consistency as a tool to improve the validity of epidemiological assessments. For the latter purpose, a software application, DISMOD II, is available from the World Health Organization (WHO) Web site.

Uses

DALYs were first employed in the 1993 World Development Report to quantify the burden of ill health in different regions of the world. The Global Burden of Disease (GBD) study, edited by Murray and Lopez and published in 1996, used a revised DALY measure. The DALY was developed to facilitate the inclusion of nonfatal health outcomes in debates on international health policy, which had often focused on child mortality, and to quantify the burden of disease using a measure that could also be used for cost-effectiveness analysis. DALYs have been widely used in global- and national-burden-of-disease studies and to assess disease control priorities. They have also been used to make the case for primary prevention programs for disorders such as stroke prevention in Australia and in assessing funding allocations in medical research programs in Australia, Canada, and the United States in relation to the burden associated with different diseases.

DALYs are also frequently used in economic evaluations of public health interventions, particularly in low- and middle-income countries. The DALY is the health effect measure that is recommended by the WHO's Choosing Interventions that are Cost Effective (WHO-CHOICE) program for generalized cost-effectiveness analysis (GCEA) and is also used in the World Bank's Disease Control Priorities in Developing Countries program. In GCEA, the costs and effectiveness of all possible interventions are compared with the null set for a group of related interventions to select the mix that maximizes health for the given resource constraints. DALYs and quality-adjusted life years (QALYs) are both health-adjusted life year (HALY) measures that use time as a common metric. QALYs were developed for the economic evaluation of clinical interventions and remain the dominant outcome measure used in cost-utility analyses that compare the costs and health effects of specific interventions using a preference-based measure of health. It is standard for cost-utility analyses using QALYs to subtract averted direct costs of care (cost offsets) from intervention costs to calculate the net cost of interventions used to calculate cost-effectiveness ratios, which can be negative. In contrast, most analyses that use DALYs do not calculate cost offsets, primarily because reliable information on such costs is extremely scarce in low- and middle-income countries.

Components

DALYs are composed of two components, years of life lost (YLL) due to premature death and years lived with disability (YLD) associated with nonfatal injuries and disease. YLL represents the stream of lost healthy life due to premature death at a particular age. It is calculated as the product of the number of deaths due to a specific cause and the years lost per death. YLD is calculated as the product of incidence of a specific cause and its average duration, multiplied by a disability or severity weight for that condition. Disability weights are assigned on a scale from 0 (representing perfect health) to 1 (representing death), in addition to an optional age-weighting parameter. The scale of DALY weights is inverted from that used to calculate QALYs. Consequently, when DALYs are used as the denominator in cost-effectiveness ratios, one refers to the cost per DALY averted as opposed to cost per QALY gained. Equivalently, the DALY is a health gap measure, whereas the QALY is a health gain measure. When different interventions are evaluated by some studies using DALYs and by others using QALYs, ranking interventions according to cost-effectiveness ratios may be possible even though there is no systematic formula for converting between the two measures, as long as the same approach is used in each study to calculate costs.

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