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DALY FAQ

What is a DALY?

A DALY is a Disability Adjusted Life Year, and is equivalent to a year of healthy life lost due to a health condition. The DALY, developed in 1993 by the World Bank, combines the years of life lost from a disease (YLL) and the years of life spent with disability from the disease (YLD). Initial attempts to estimate the burden of diseases concentrated on mortality (the number of deaths in a given time or place) only, disregarding the non-fatal burden of many diseases including infectious diseases. The DALY is widely used today by organizations such as the WHO and the NIH.

DALYs count the gains from both mortality (how many more years of life lost due to premature death are prevented) and morbidity (how many years or parts of years of life lost due to disability are prevented). The advantages of the DALY are that it is a metric that is recognized and understood by external audiences. It helps PSI gauge its contribution relative to overall burden of disease by geographic region or health area. Combined with cost data, DALYs allow PSI to regularly and respectively estimate the cost-effectiveness of its interventions in different countries.

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How are DALYs calculated?

DALYs averted through PSI interventions are derived through a model called the DALY calculator. Its purpose is to translate inputs — in the form of sales or services or health communications, into outputs — in this case in the form of DALYs. The model attempts to take the estimated impact of a product such as a condom or a malaria bed net, and translate it into a widely accepted measure of health outcome.

The DALY calculator is based on the latest available epidemiological data; estimates of the impact of health conditions from WHO; best practices in public health; and advice from a diverse set of experts that reviewed and commented on earlier versions. One thing a model rarely does is replicate reality exactly, but it tries to develop a version of reality that incorporates as many of the key factors as possible, and probably more importantly the relationships between them. The development of any model is an evolutionary process, and PSI plans to continually refine and improve the calculator.

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Where do the assumptions come from?

One of the main problems with data on health outcomes is a lack of information from the developing world. Some diseases have been researched thoroughly while very little knowledge exists for others. This is a challenge to deriving common assumptions. PSI uses the Disease Control Priorities Project 2 (DCP2) data for projections of the impact of diseases and health conditions in the regions in which it operates. The DCP2 project gives a thorough and complete assessment of the state of global health by world region and is a source of internationally comparable and consistent health data.

However, because of the issue of co-morbidity (the effect of such additional disorders or diseases), DCP2 data usually cites lower ill-health and death rates than those used by other authorities such as the WHO. For example, someone with HIV is likely to suffer and die from diarrhea six times more often than someone without. But if they do die, is HIV or diarrhea the cause of death? Indeed, HIV never killed anyone - it is always associated conditions that lead to death - but to say that HIV is non-fatal is highly misleading. To counter this, DCP2 scales back on morbidity and mortality claims in order to avoid double or triple counting. All mortality statistics in the DALY are derived from the DCP2. However, DCP2 is not explicit about morbidity, so where this is the case, we used statistics from other sources including UNICEF, WHO, World Bank and others.

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What are the disadvantages of DALYs?

Some have argued that DALYs ‘overweight’ death and do not give enough credit to quality of life interventions such as safe water or nutrition. Few would argue that water or nutrition interventions are unimportant, but DALYs will encourage certain priorities. In the allocation of scarce resources, interventions which save lives should arguably take urgency over those that don’t even though both may be important. To ensure consistency and credibility for internal and external audiences, PSI uses the ’weight’ or likelihood of death or disability due to any particular disease established by the World Health Organization and used by others that estimate DALYs.

We acknowledge the imperfections of the DALY. A common critique of DALYs is that they use age weighting and value the lives of those who are economically active over the lives of the young and the old. PSI believes that this critique is valid and therefore does not do age weighting in its DALY calculator – all lives are treated equal.

Also, many say that DALYs ignore the tangential and social impacts of ill health, a critique that may be valid but very difficult to resolve. For example, a woman with diarrhea may not be able to travel to market to sell her vegetables, and hence has no money to send her children to school. DALYs would only measure the impact on the woman herself, not her family.

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How does the DALY calculator determine how targeted an intervention or program is?

The DALY calculator is sensitive to targeting. Interventions targeting populations at highest risk of infection or disease are more impactful and prevent more disease and death than those provided to the population at large. For example, Oral Rehydration Salts (ORS) is an efficient DALY generator because it is a self-targeting product – you only use it if you already have diarrhea. The same targeting issue applies to nets; they are more often used by kids than adults.

Targeted goods get much more return; if you serve rural kids with nets you get a big bang for your buck, but if nets are being used by urban adults you get virtually nothing.
DALYs reward targeting, since DALY cares if the eventual outcome of the intervention is not just that the consumer is protected, but that death or disease has actually been averted. Currently, the DALY calculator rewards targeting of condoms to groups at high risk of HIV infection and targeting of malaria bed nets to rural consumers.

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With access to safe water becoming such an important health and quality of life issue, why are DALYs averted from water purification products low?

Diarrhea is a big killer but it is hard to target for prevention. Most people who get diarrhea are at little to no risk of death; only children under the age of five are high risk. PSI DALY model assumes that PSI provision of water purification products is slightly targeted but that most is used by the overall household and thus averts little death or serious morbidity. Given that, only a small proportion of PSI provision of these products is preventing death — you would have to sell a very large amount of water purification products to reach a significant amount to the children.

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Is averting death really all that matters under DALYs?

DALY regards a healthy year as worth 100% of a year. If you are sick, you ‘lose’ part of that year, the size of the loss being determined by the severity of the condition. For example, diarrhea in an adult ‘loses’ about a week. But if the person recovers, the future years are still intact. However, if the person dies, he/she ‘loses’ the year in which they die and all the years into the future that they would otherwise have lived (discounted over time, which is the convention in DALYs – discounts allow for uncertainty of the future and the further you go into the future, the heavier the discount.)

Thus, averting a death generates many more DALYs than averting sickness. Therefore, interventions that prevent death are well-rewarded while interventions that prevent sickness or debilitation are therefore far less efficient at producing DALYs.

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Can we compare the cost effectiveness across interventions and countries?

We can compare an HIV program in Kenya to an HIV program in Nepal – but the playing field is not level because conditions vary by country. It is harder to avert HIV DALYs in Burkina Faso than in Zimbabwe, for instance, because Burkina has low HIV prevalence and lots of protective male circumcision, the opposite of what prevails in Zimbabwe.

We can also compare an HIV intervention in Kenya to a malaria intervention in Myanmar dollar for dollar.

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Can we compare PSI cost effectiveness to that of others who use DALYs?

Yes, but with care. There are many potential ways to calculate DALYs and we have to be sure that the methodologies are comparable and we are comparing apples to apples.

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What kinds of interventions do DALYs favor, or what kinds of interventions do I need to generate more DALYs?

DALYs favor interventions that save the most lives and those that target those at highest risk of infection or disease. DALYs will incentivize country programs to address health problems that are highly prevalent in their country, do better targeting of most at-risk populations, and measure their targeting.

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Does PSI use a country-specific life expectancy or a global life expectancy?

Currently, PSI is using a global life expectancy for its platforms (i.e., 81.25 years). The DALY is defined as a measure of health gap between current health status and an ideal situation where everyone lives into old age free from disease and disability (Global Burden of Disease and Risk Factors, World Bank 2007; page 3). It uses a global ‘ideal’ standard life expectancy for all population subgroups, whether or not their current life expectancy was lower than that of other groups (Global Burden of Disease and Risk Factors, World Bank 2007; page 49). The argument is based on equity considerations. For example, if a program averted 10 HIV infections. So, if it is conducted in Japan, 10 infections are averted; if in Russia, 10 infections are averted as well. However, Japan will averted more DALYs than Russia just because the current life expectancy of Japanese is higher if we use real life expectancies. We are at risk of drawing a wrong conclusion that doing the program in Japan but not in Russia will generate more impact.

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