Interpreting the Disability-Adjusted Life-Year (DALY) metric

What is a DALY?

The Disability-Adjusted Life-Year (DALY) is a metric that combines the burden of mortality and morbidity (non-fatal health problems) into a single number.1 It is the primary metric used by the World Health Organization to assess the global burden of disease,2 and the primary metric used by projects such as the Disease Control Priorities in Developing Countries report3 to quantify the cost-effectiveness of different programs. We fully explain the formula and meaning of the DALY metric in a series of posts on our blog.

What are the limitations of the DALY metric? Why does GiveWell not focus exclusively on it?

The DALY metric is used to provide a single number to capture all of the health costs caused by a disease (or averted by an aid program). 1 DALY could represent 1 year of life lost (due to early death), 1.67 years spent with blindness, 5.24 significant malaria episodes, 41.67 years spent with intestinal obstruction due to ascariasis (a parasite), or many possible combinations of these and other symptoms.4 There is no way of knowing, from just how many DALYs a program is said to have averted, whether it has saved lives, prevented large numbers of minor health problems, or some combination thereof. We feel that this creates a number of problems for donors seeking the charity that best fits their values. More in our blog series on DALYs, available here.

Interpreting the DALY metric

The Disease Control Priorities report frequently estimates the cost-effectiveness of different programs in terms of cost per DALY averted. We seek to convert these figures, using fairly minimal assumptions, into numbers that have clearer and more intuitive meanings, including:

• Years of a given symptom (blindnesss, skin disease, etc.) averted
• Cases of a given health condition (blindness, skin disease, etc.) averted

In some cases, our conversion calculation is simple enough to explain fully in a footnote. In other cases, we use a calculation that is still simple but has several steps and is shared across many estimates, so we use the following spreadsheet:

• DalysToSymptoms.xls (Excel 2000 format)

Sources

• GiveWell. Blog series on the DALY metric
• GiveWell. DALYs to symptoms (XLS).
• Jamison, Dean T. et al., eds. 2006. Disease control priorities in developing countries (2nd Edition) (PDF). New York: Oxford University Press.
• Lopez, Alan D. et al., eds. 2006. Global burden of disease and risk factors (PDF). New York: Oxford University Press.
• World Health Organization. Global burden of disease (GBD). http://www.who.int/healthinfo/global_burden_disease/en/index.html (accessed April 15, 2010). Archived by WebCite® at http://www.webcitation.org/5p118giwH.

• 1. "The DALY, developed for the GBD study,is an example of a health gap indicator that extends the notion of mortality gaps to include time lived in states other than excellent health." Lopez et al. 2006, Pg 48.
• 2. "The overall burden of disease is assessed using the disability-adjusted life year (DALY), a time-based measure that combines years of life lost due to premature mortality and years of life lost due to time lived in states of less than full health. The WHO GBD project draws on a wide range of data sources to quantify global and regional effects of diseases, injuries and risk factors on population health." World Health Organization, "Global Burden of Disease (GBD)."
• 3. DALYs are referred to throughout the Disease Control Priorities report (Jamison et al. 2006) as a measure of disease burden. See examples of its prominent use on Pgs 31-2, Tables 1.A1 and 1.A2 and Table 1.3.
• 4. These numbers are based on the DALY formula (described with references on our blog) and the official Disability Weights found in Lopez et al. (2006), Pgs 119-123.