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QALYS and DALYS
If we're trying to find those charities that do the most to help people in the developing world, we need some way of measuring and comparing potential benefits. There doesn't exist a single, go-to measure that's readily applicable across domains as diverse as sanitation, economic empowerment, and political change. However, there do exist well-regarded methods for measuring and comparing the value of health improvements: these are measurable in terms of Health Adjusted Life Years (HALYs). 1 As most of our work in charity evaluation has focused on global health, we have largely relied on this kind of metric. We use this page to explain, in broad outline, how it works.
Gold, Marthe R., David Stevenon, and Dennis G. Fryback (2002) “HALYs and QALYs and DALYs, Oh My: Similarities and Differences in Summary Measures of Population Health.” Annual Review of Public Health 23, 115-134
HALYs in General
At the most basic level, a Health Adjusted Life Year (HALY) represents a year of life, adjusted in value, so that a year lived in poor health is less valuable than a year lived in full health (all else being equal). We can use a HALY metric to measure the harm and benefit caused by different diseases and the interventions designed to reduce them, capturing both quality and quantity of life lost/gained.
There exist two different HALY metrics: Quality Adjusted Life Years (QALYs) and Disability Adjusted Life Years (DALYs). These were designed by researchers working with different aims, and so diverge on a number of points. Continual efforts are made to improve these measures,2 and they evolve over time: members of Giving What We Can have been involved in designing recent adjustments to the DALY metric. 3
On the future of the QALY measure, see the special issue Building a Pragmatic Road: Moving the QALY Forward, Value in Health 12, Supplement 1, 2009
GWWC members Nick Beckstead, Nir Eyal, Toby Ord, and Thomas Pogge were amongst philosophers asked to advise on ethical aspects of the DALY measure at the recent Critical Ethical Choices for DALYs meeting, convened by the World Health Organisation and World Bank in preparation for publication of the 2010 Global Burden of Disease Report.
As the QALY metric is the most straightforward and intuitive, we'll begin by explaining how it works. The QALY system was developed by economists and psychologists in the 1960s as a tool for measuring the cost-utility ratio of health interventions: roughly, the degree of health improvement you can 'buy' for a population, at a given price. The QALY measure is widely used for setting health priorities in the developed world, having been taken up by the National Institute for Clinical Health and Excellence (NICE) in the UK and the Panel on Cost-Effectiveness in Health and Medicine in the USA.
In the QALY system, health is scored from 0, representing death, to 1, representing full health. The value of additional years of life is determined by multiplying the years added by the value of the health state: one additional year of life lived at full health represents one QALY, as do two additional years lived with a health score of 0.5, and so on. Health improvements that do not extend life are also measurable in terms of QALYs, by multiplying the value of the improvement over the years to which it applies: so, if someone's health is improved from 0.5 to 1 over a period of two years, this gets valued as one QALY.
How are these health scores assigned? In the QALY system, they are typically not assigned to specific disease conditions, such as angina or anaemia. Instead, they are assigned to health states understood in more holistic terms. For example, under the EuroQOL 5-Item Scale , health states are conceived in terms of the extent to which people are able to move about freely, care for themselves, and carry out their usual activities without pain or anxiety.4 A disability weight between 0 and 1 is typically assigned by eliciting the preferences of a representative sample of the population. For example, using the standard gamble, people may be asked what probability of dying they would be willing to accept for an intervention that could improve their health in some way; using the time trade-off method , people are asked to what extent they would be willing to forego years of life in return for health improvements of various kinds. 5
Other means of assigning weights are used, including visual analogue scales and person trade-off decisions. For further information on the full range of techniques available, see: Weinstein, Milton C., George Torrance, and Alistair McGuire (2009) “QALYs: The Basic.” Value in Health 12, S5-S9
Whereas QALYs were designed to measure health benefits, DALYs were designed by the World Bank and World Health Organisation in the 1990s to measure a negative: the global burden of disease. 6 Under this system, benefits have to be thought of in terms of reductions in disease burden . DALYs are used more often than QALYs to set health priorities in the developing world: for example, they are the standard tool of the Disease Control Priorities Project, whose DPC2 report is one of our key sources. Very roughly, one might think of a DALY as the inverse of a QALY. 7 Since DALYs are designed to quantify a negative, a DALY represents a year of life at full health that is lost. To measure the value of non-fatal outcomes, health states are assigned a disability weight ranging from 1 to 0, where 0 represents full health. Using the DALY metric, benefit may be measured in terms of 'DALYs averted'.
ee Murray, Christopher J.L. and Alan D.Lopez, eds. (1996). The Global Burden of Disease: a comprehensive assessment of mortality and disability from diseases, injuries, and risk factors from 1990 and projected to 2020. Cambridge, MA: Harvard University Press.
In reality, converting between QALYs and DALYs is far from straightforward, and using one or the other measure can sometimes lead to different conclusions in priority setting. See, Gold Marthe R. and Peter Muennig (2002) “Measure-dependent variation in burden of disease estimates: implications for policy.” Medical Care 40, 260-266
When a condition is fatal, how do we determine the number of years lost in death? To do so, we have to pick a benchmark for life-expectancy, relative to which the person falls short. A universal, standardized life-expectancy table is developed for this purpose: this table allows us to say, for example, that a death occurring at age 40 involves 46.64 years lost. 8 Note that life-expectancies assigned using this table do not vary by society, although actual life-expectancy does: the choice to hold life-expectancy constant was made on egalitarian grounds, so that the death of a person aged 40 in Kenya was not counted as less valuable to prevent than the death of a person aged 40 living in the UK. 9 As a general rule, Giving What We Can uses a life expectancy of 60 to convert saving the life of a young child in the developing world into a DALY measure. This is a typical life-expectancy at birth for the developing countries in which our charities operate. Since we look only at developing world charities, the egalitarian concerns noted above do not apply.
This figure reflect the table used in GBD 2010. See: Murray, Christopher J.L., Majid Ezzati, Abraham D. Flaxma, et al. (2012) “GBD 2010: design, definitions, and metrics.” The Lancet 380, 2063–66
On the egalitarian presuppositions of the DALY measure, see: Murray, Christopher J.L. (1996) “Rethinking DALYs” in Murray, Christopher J.L. and Alan D.Lopez, eds. (1996). The Global Burden of Disease: a comprehensive assessment of mortality and disability from diseases, injuries, and risk factors from 1990 and projected to 2020. Cambridge, MA: Harvard University Press, 1-98
Under the DALY system, a disability burden is assigned to a specific disease state, such as angina or anaemia. These weights were originally assigned based on the judgement and deliberation of experienced health-care professionals, using a method known as the person trade-off protocol: this requires making decisions about how to allocate additional years of life and/or health improvements across different groups varying in their level of health. 10 More recently, weights have been assigned by relying on the judgement of representative samples drawn from the general public in Bangladesh, Indonesia, Peru, Tanzania, and the USA: these primarily involve paired comparisons, in which people are asked to say which of two hypothetical individuals in different health states they regard as most healthy. 11
See: ibid., 22-43
Salmon, Joshua A., Theo Vos, Daniel R Hogan, et al. (2012) “Common values in assessing health outcomes from disease and injury: disability weights measurement study for the Global Burden of Disease Study 2010.” The Lancet 380, 2129-2143
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