Disability-Adjusted Life Year

Disability-Adjusted Life Year in Burden of Disease and Cost-Effectiveness Analysis

One DALY is one lost year of healthy life, so the measure puts deaths and non-fatal illness on one scale, so that causes as different as neonatal disorders and low back pain can be ranked by the healthy life they take. It is the summary measure of the Global Burden of Disease (GBD) study and of the World Health Organization (WHO) Global Health Estimates, and cost per DALY averted is the preferred outcome of cost-effectiveness analysis in low- and middle-income settings. The calculation has changed since the measure was introduced in 1993, so DALY figures from different GBD rounds, or from a burden study and an economic evaluation, are not automatically comparable. Unlike the quality-adjusted life year (QALY), it counts health lost rather than health gained, which is why interventions are credited with DALYs averted.

Origins in the first Global Burden of Disease study

The first GBD study, commissioned by the World Bank for its World Development Report 1993 and carried out by the Harvard School of Public Health and WHO, introduced the DALY as a single measure of the burden of diseases, injuries and risk factors. WHO describes the DALY as a generalisation of potential years of life lost to include lost good health, and states that one DALY can be thought of as one lost year of healthy life. Measured burden is therefore the gap between a population's health and that of a normative reference population, not a count of people who are ill.

Murray's 1994 technical paper set out four social choices in the original measure. Years lost to a death came from a standard based on model life table West Level 26, time lived at different ages was valued with an exponential age-weighting function, time lived with disability was made comparable with time lost to death through six severity classes weighted between 0 and 1, and a 3% discount rate was applied.

Adding years of life lost and years lived with disability

A DALY total for a cause is the sum of a fatal and a non-fatal component. WHO calculates both for the same cause, sex, age group and year.

$$DALY_{c} = YLL_{c} + YLD_{c}$$

where $DALY_{c}$ is the DALYs from cause $c$, $YLL_{c}$ is the years of life lost to deaths from that cause and $YLD_{c}$ is the years lived with disability from it. Each component has its own formula.

$$YLL_{c} = \sum_{a} N_{c,a} \times L_{a}, \qquad YLD_{c} = \sum_{s} P_{s} \times DW_{s}$$

where $N_{c,a}$ is the number of deaths from cause $c$ at age $a$, $L_{a}$ is the standard remaining life expectancy at that age, $P_{s}$ is the number of people living with sequela $s$ of the cause and $DW_{s}$ is its disability weight. WHO's standard for its 2000 to 2019 estimates gives a life expectancy at birth of 90 years.

What changed in GBD 2010 and why rounds differ

After a consultation in July 2011 with 21 philosophers, ethicists and economists, GBD 2010 and later rounds simplified the measure. They adopted a new normative standard life table, calculated YLDs as prevalence times disability weight, adjusted for comorbidity, and dropped discounting and unequal age weighting. WHO adopted the same simplified method after consultations in late 2012, with its own standard life table.

WHO reports that the move to a 0% discount rate and no age weighting substantially increased the number of DALYs and raised the share falling at younger and older ages. With new data and revised methods as well, WHO states that its estimates for 2000 to 2019 are not directly comparable with its earlier DALY estimates.

Global DALY estimates from GBD 2021

GBD 2021 estimated 2.88 billion DALYs worldwide in 2021, up from 2.63 billion in 2010. The authors attribute much of the rise to population growth and ageing, as the global age-standardised DALY rate fell by 14.2% between 2010 and 2019, before rising again in 2020 and 2021. Of the 2021 total, 1.98 billion DALYs (68.7%) were YLLs and 907 million (31.3%) were YLDs.

COVID-19 was the leading cause of DALYs in 2021, with 212.0 million, followed by ischaemic heart disease (188.3 million), neonatal disorders (186.3 million) and stroke (160.4 million). Counts show where health loss is concentrated, while age-standardised rates remove the effects of population size and age structure.

DALYs averted as an outcome in cost-effectiveness analysis

A burden study counts the health lost in a population in a year. An economic evaluation instead models the DALYs a cohort would incur with and without an intervention over a time horizon, and the difference is the DALYs averted. The incremental cost-effectiveness ratio then divides the extra cost by that difference.

$$ICER = \frac{C_{1} - C_{0}}{DALY_{0} - DALY_{1}}$$

where $C_{1}$ and $C_{0}$ are the costs with and without the intervention and $DALY_{0}$ and $DALY_{1}$ are the DALYs incurred without and with it, so the denominator is DALYs averted.

Fox-Rushby and Hanson defined DALYs for this purpose as the present value of future years of life lost and of future years lived with disability. They found that many published analyses could not be compared or transferred because their assumptions differed or were unclear, and proposed minimum reporting criteria.

WHO's Choosing Interventions that are Cost-Effective programme (WHO-CHOICE) evaluates interventions over a lifetime horizon. It presents health benefits undiscounted and discounted at 3%, with costs discounted at 3%, and reports cost per healthy life year gained to separate this gain measure from the GBD loss measure. It keeps the cost per DALY metric because it is the preferred outcome in low- and middle-income settings and because no single database of QALY weights covers every disease state and country. A review by Neumann and colleagues found 6,438 cost-per-QALY and 543 cost-per-DALY studies published through 2016, with cost-per-DALY studies concentrated on infectious diseases and on low and lower-middle income countries.

GDP-based thresholds and WHO's current position

In 2001 WHO's Commission on Macroeconomics and Health suggested cost-effectiveness thresholds based on multiples of a country's per capita gross domestic product (GDP). In 2005 authors writing for WHO-CHOICE called interventions very cost-effective if they averted a DALY for less than average per capita income, and cost-effective below three times that income. Bertram and colleagues, writing for WHO in 2016, stated that none of these thresholds should be used alone as a decision rule, and the 2021 WHO-CHOICE update recommends against using a threshold as a decision rule at all, presenting results in league tables instead.

In England the NICE manual (PMG36) asks for health effects to be expressed in QALYs and refers to maximum acceptable ICERs of GBP 25,000 to GBP 35,000 per QALY gained. A cost per DALY averted should not be read directly against a cost per QALY threshold, since the two measures use different weights and different life expectancies.

Worked example of DALYs averted by a treatment programme

All figures are illustrative. A district has a cohort of 200 adults aged 40 with a condition that, without treatment, kills 20 of them at once and leaves the 180 survivors with 10 years at a disability weight of 0.3, after which they recover fully. A treatment programme cuts deaths to 8 and lowers the weight for all 192 survivors to 0.1 for the same 10 years, at an extra cost of USD 444,000. The standard remaining life expectancy at age 40 is taken as 45 years.

1. DALYs without the programme. The deaths give YLLs and the survivors give YLDs.

$$DALY_{0} = 20 \times 45 + 180 \times 0.3 \times 10 = 900 + 540 = 1440$$

where 900 is the YLLs from 20 deaths and 540 is the YLDs of 180 survivors over 10 years.

2. DALYs with the programme. The 12 people whose deaths are prevented join the survivors and so add to the YLDs.

$$DALY_{1} = 8 \times 45 + 192 \times 0.1 \times 10 = 360 + 192 = 552$$

where 360 is the YLLs from 8 deaths and 192 is the YLDs of 192 survivors.

3. Cost per DALY averted. The programme averts 1,440 less 552, or 888 DALYs, of which 540 are YLLs averted (61%) and 348 are YLDs averted, net of the 12 YLDs added by the people saved.

$$ICER = \frac{444000}{1440 - 552} = \frac{444000}{888} = 500$$

where the numerator is the extra cost in USD, so the programme costs USD 500 per DALY averted, undiscounted.

4. Discounting at 3% a year. With continuous discounting a stream of $L$ years has a present value of $(1 - e^{-rL})/r$, which at $r = 0.03$ gives 24.692 for 45 years and 8.639 for 10 years.

$$DALY_{0} = 20 \times 24.692 + 180 \times 0.3 \times 8.639 = 493.84 + 466.51 = 960.35$$

where the discounted factors replace 45 and 10 years. With the programme the same factors give 197.54 + 165.87 = 363.41, so DALYs averted fall to 596.94 and the cost per DALY averted rises to about USD 744 (444,000 divided by 596.94). YLLs averted now supply about 50% of the gain (296.30 of 596.94), down from 61%, because the years they save lie further in the future than the relief from lower severity.

5. Local survival. If the 12 people saved would live 30 more years in their own setting, a model following their expected survival credits 360 life-years (12 times 30) for the deaths prevented, not the 540 implied by the standard, so the life expectancy and discount rate used should both be reported.

How DALYs differ from QALYs

The DALY was built to describe burden, the QALY to value health gains in economic evaluation. Neumann and colleagues note that QALY weights run from 0 for dead to 1 for perfect health and reflect health state attributes, whereas DALY weights run the other way, with 1 for dead and 0 for perfect health, and each corresponds to a specific disease.

Sassi showed that the relation between QALYs gained and DALYs averted depends on age of onset, duration and the two sets of weights, with more factors in play for fatal diseases. Airoldi and Morton found that the two measures rank interventions systematically differently, and traced this to the DALY's use of life expectancy tables rather than to framing health as gain or loss, as step 5 shows.

Limitations and common misreadings

The DALY rests on several conventions, and most errors come from reading it as more than it is. Critiques of the weights themselves are covered on the disability weight page.

• Equity. Arnesen and Nord argued that the DALY values life-extending programmes for disabled people less than the same programmes for people without disability, since a death prevented for someone living with a disability averts fewer DALYs, as step 2 shows.
• Burden is not priority. A large burden does not show that effective and affordable interventions exist, and WHO warns that data gaps in high-mortality regions call for caution when interpreting global comparisons.
• Mixing conventions. A burden DALY is undiscounted and uses a standard life table, while an evaluation may discount and use local survival, so the two should not be compared until one is restated.
• Reading a DALY as a QALY lost. Utility is not simply one minus the disability weight, because QALY weights value health state attributes while each disability weight corresponds to a specific disease.

Sources

• Airoldi M, Morton A. Adjusting life for quality or disability: stylistic difference or substantial dispute? Health Economics. 2009;18(11):1237-1247. doi:10.1002/hec.1424
• 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
• Bertram MY, Lauer JA, De Joncheere K, Edejer T, Hutubessy R, Kieny MP, Hill SR. Cost-effectiveness thresholds: pros and cons. Bulletin of the World Health Organization. 2016;94(12):925-930. doi:10.2471/BLT.15.164418
• Bertram MY, Lauer JA, Stenberg K, Tan Torres Edejer T. Methods for the economic evaluation of health care interventions for priority setting in the health system: an update from WHO CHOICE. International Journal of Health Policy and Management. 2021;10(11):673-677. doi:10.34172/ijhpm.2020.244
• Fox-Rushby JA, Hanson K. Calculating and presenting disability adjusted life years (DALYs) in cost-effectiveness analysis. Health Policy and Planning. 2001;16(3):326-331. doi:10.1093/heapol/16.3.326
• GBD 2021 Diseases and Injuries Collaborators. Global incidence, prevalence, years lived with disability (YLDs), disability-adjusted life-years (DALYs), and healthy life expectancy (HALE) for 371 diseases and injuries in 204 countries and territories and 811 subnational locations, 1990-2021: a systematic analysis for the Global Burden of Disease Study 2021. The Lancet. 2024;403(10440):2133-2161. doi:10.1016/S0140-6736(24)00757-8
• Murray CJL. Quantifying the burden of disease: the technical basis for disability-adjusted life years. Bulletin of the World Health Organization. 1994;72(3):429-445.
• National Institute for Health and Care Excellence. NICE technology appraisal and highly specialised technologies guidance: the manual (PMG36). London: NICE; 2022, last updated 13 October 2026. Chapter 4.
• Neumann PJ, Anderson JE, Panzer AD, Pope EF, D'Cruz BN, Kim DD, Cohen JT. Comparing the cost-per-QALYs gained and cost-per-DALYs averted literatures. Gates Open Research. 2018;2:5. doi:10.12688/gatesopenres.12786.2
• Sassi F. Calculating QALYs, comparing QALY and DALY calculations. Health Policy and Planning. 2006;21(5):402-408. doi:10.1093/heapol/czl018
• World Health Organization. WHO methods and data sources for global burden of disease estimates 2000-2019. Global Health Estimates Technical Paper WHO/DDI/DNA/GHE/2020.3. Geneva: WHO; 2020. Sections 1, 2 and 5.