The disability-adjusted life year (DALY) is a measure of overall
disease burden, expressed as the number of years lost due to
ill-health, disability or early death. It was developed in the 1990s
as a way of comparing the overall health and life expectancy of
The DALY is becoming increasingly common in the field of public health
and health impact assessment (HIA). It "extends the concept of
potential years of life lost due to premature death... to include
equivalent years of 'healthy' life lost by virtue of being in states
of poor health or disability." In so doing, mortality and morbidity
are combined into a single, common metric.
1.1 Age weighting
2 Economic applications
3.3 PTSD rates
4 History and usage
6 See also
8 External links
The disability-adjusted life year is a societal measure of the disease
or disability burden in populations. DALYs are calculated by combining
measures of life expectancy as well as the adjusted quality of life
during a burdensome disease or disability for a population. DALYs are
related to the quality-adjusted life year (QALY) measure; however
QALYs only measure the benefit with and without medical intervention
and therefore do not measure the total burden. Also, QALYs tend to be
an individual measure, and not a societal measure.
Traditionally, health liabilities were expressed using one measure,
the years of life lost (YLL) due to dying early. A medical condition
that did not result in dying younger than expected was not counted.
The years lost due to disability (YLD) component measures the burden
of living with a disease or disability.
DALYs are calculated by taking the sum of these two components:
DALY = YLL + YLD
The DALY relies on an acceptance that the most appropriate measure of
the effects of chronic illness is time, both time lost due to
premature death and time spent disabled by disease. One DALY,
therefore, is equal to one year of healthy life lost.
How much a medical condition affects a person is called the disability
weight (DW). This is determined by disease or disability and does not
vary with age. Tables have been created of thousands of diseases and
disabilities, ranging from Alzheimer's disease to loss of finger, with
the disability weight meant to indicate the level of disability that
results from the specific condition.
Alzheimer's and other dementias
AIDS, not on ART
Burns 20%-60% of body
Moderate depression episode
Amputation of foot
Amputation of finger
Low back pain
Examples of the disability weight are shown on the right. Some of
these are "short term" and the long-term weights may be different.
The most noticeable change between the 2004 and 2010 figures for
disability weights above are for blindness as it was considered the
weights are a measure of health rather than wellbeing (or welfare) and
a blind person is not considered to be ill. "In the GBD terminology,
the term disability is used broadly to refer to departures from
optimal health in any of the important domains of health."
At the population level, the disease burden as measured by DALYs is
calculated by adding YLL to YLD. YLL uses the life expectancy at the
time of death. YLD is determined by the number of years disabled
weighted by level of disability caused by a disability or disease
using the formula:
YLD = I x DW x L
In this formula I = number of incident cases in the population, DW =
disability weight of specific condition, and L = average duration of
the case until remission or death (years). There is also a prevalence
(as opposed to incidence) based calculation for YLD. Premature death
is calculated by YLL = N x L, where N = number of deaths due to
condition, L = standard life expectancy at age of death (expectancy
– age at death).
Japanese life expectancy statistics are used as the standard for
measuring premature death, as the Japanese have the longest life
Some studies use DALYs calculated to place greater value on a year
lived as a young adult. This formula produces average values around
age 10 and age 55, a peak around age 25, and lowest values among very
young children and very old people.
A crucial distinction among DALY studies has been the use of
"age-weighting", in which the value of each year of life depends on
age; however, the
World Health Organization
World Health Organization has abandoned age
weighting and time discounting in DALYs since 2010.
There are two components to this differential accounting of time:
age-weighting and time-discounting. Age-weighting is based on the
theory of human capital. Commonly, years lived as a young adult are
valued more highly than years spent as a young child or older adult,
as these are years of peak productivity. Age-weighting receives
considerable criticism for valuing young adults at the expense of
children and the old. Some criticize, while others rationalize, this
as reflecting society's interest in productivity and receiving a
return on its investment in raising children. This age-weighting
system means that somebody disabled at 30 years of age, for ten years,
would be measured as having a higher loss of DALYs (a greater burden
of disease), than somebody disabled by the same disease or injury at
the age of 70 for ten years.
This age-weighting function is by no means a universal methodology in
HALY studies, but is common when using DALYs. Cost-effectiveness
studies using QALYs, for example, do not discount time at different
ages differently. This age-weighting function applies only to the
calculation of DALYs lost due to disability. Years lost to premature
death are determined from the age at death and life expectancy.
Global Burden of Disease Study (GBD) 2001–2002 counted
disability adjusted life years equally for all ages, but the GBD 1990
and GBD 2004 studies used the formula
displaystyle W=0.1658Ye^ -0.04Y
is the age at which the year is lived and
is the value assigned to it relative to an average value of 1.
In these studies future years were also discounted at a 3% rate to
account for future health care losses. Time discounting, which is
separate from the age-weighting function, describes preferences in
time as used in economic models.
The effects of the interplay between life expectancy and years lost,
discounting, and social weighting are complex, depending on the
severity and duration of illness. For example, the parameters used in
the GBD 1990 study generally give greater weight to deaths at any year
prior to age 39 than afterward, with the death of a newborn weighted
at 33 DALYs and the death of someone aged 5–20 weighted at
approximately 36 DALYs.
As a result of numerous discussions, by 2010 the World Health
Organization had abandoned the ideas of age weighting and time
discounting. They had also substituted the idea of prevalence for
incidence (when a condition started) because this is what surveys
The methodology is not an economic measure. It measures how much
healthy life is lost. It does not assign a monetary value to any
person or condition, and it does not measure how much productive work
or money is lost as a result of death and disease. However, HALYs,
including DALYs and QALYs, are especially useful in guiding the
allocation of health resources as they provide a common numerator,
allowing for the expression of utility in terms of DALYs/dollar, or
QALY/dollar. For example, in Gambia, provision of the pneumococcal
conjugate vaccine costs $670 per DALY saved. This number can then
be compared to other treatments for other diseases, to determine
whether investing resources in preventing or treating a different
disease would be more efficient in terms of overall health.
Schizophrenia has a 0.53 weighting and a broken femur a 0.37 weighting
in the latest WHO weightings.
Cancer (25.1/1,000), cardiovascular (23.8/1,000), mental problems
(17.6/1,000), neurological (15.7/1,000), chronic respiratory
(9.4/1,000) and diabetes (7.2/1,000) are the main causes of good years
of expected life lost to disease or premature death. Despite this,
Australia has one of the longest life expectancies in the world.
These illustrate the problematic diseases and outbreaks occurring in
2013 in Zimbabwe, shown to have the greatest impact on health
disability were typhoid, anthrax, malaria, common diarrhea, and
Posttraumatic stress disorder
Posttraumatic stress disorder (PTSD) DALY estimates from 2004 for the
world's 25 most populous countries give Asian/Pacific countries and
the United States as the places where PTSD impact is most concentrated
(as shown here).
Noise-Induced Hearing Loss
The disability-adjusted life years attributable to hearing impairment
for noise-exposed U.S. workers across all industries was calculated to
be 2.53 healthy years were lost annually per 1,000 noise-exposed
workers. Workers in the mining and construction sectors lost 3.45 and
3.09 healthy years per 1,000 workers, respectively. Overall, 66% of
the sample worked in the manufacturing sector and represented 70% of
healthy years lost by all workers.
History and usage
Originally developed by
Harvard University for the
World Bank in 1990,
World Health Organization
World Health Organization subsequently adopted the method in 1996
as part of the Ad hoc Committee on Health Research "Investing in
Health Research & Development" report. The DALY was first
conceptualized by Murray and Lopez in work carried out with the World
Health Organization and the
World Bank known as the Global Burden of
Disease Study, which was published in 1990. It is now
a key measure employed by the
United Nations World Health Organization
in such publications as its Global Burden of Disease.
The DALY was also used in the 1993 World Development Report.:x
Although some have criticized DALYs as essentially an economic measure
of human productive capacity for the affected
individual,[irrelevant citation] this is not so. DALYs do have an
age-weighting function that has been rationalized based on the
economic productivity of persons at that age, but health-related
quality of life measures are used to determine the disability weights,
which range from 0 to 1 (no disability to 100% disabled) for all
disease. These weights are based not on a person's ability to work,
but rather on the effects of the disability on the person's life in
general. This is why mental illness is one of the leading diseases as
measured by global burden of disease studies, with depression
accounting for 51.84 million DALYs. Perinatal conditions, which affect
infants with a very low age-weight function, are the leading cause of
lost DALYs at 90.48 million. Measles is fifteenth at 23.11
Some commentators have expressed doubt over whether the disease burden
surveys (such as EQ-5D) fully capture the impacts of mental illness,
due to factors including ceiling effects.
Bhutan GNH Index
Broad measures of economic progress
Full cost accounting
Green national product
Green gross domestic product
Green gross domestic product (Green GDP)
Gender-related Development Index
Genuine Progress Indicator
Genuine Progress Indicator (GPI)
Global Peace Index
Gross National Happiness
Gross National Well-being (GNW)
Happy Planet Index
Happy Planet Index (HPI)
Human Development Index
Human Development Index (HDI)
ISEW (Index of sustainable economic welfare)
Progressive utilization theory
Legatum Prosperity Index
Living planet index
Law of Social Cycle
Millennium Development Goals
Millennium Development Goals (MDGs)
Subjective life satisfaction
World Values Survey (WVS)
World Happiness Report
Quality-adjusted life year
Quality-adjusted life year (QALY)
Healthy Life Years
Seven Ages of Man
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