We’ve had many discussions in the comments about the metric known as Disability-Adjusted Life-Years (DALYs). The DALY essentially converts the burdens imposed by all health issues – from premature death to blindness to injuries – into a single, consistent unit. It is the metric of choice for the Disease Control Priorities Project as well as a centerpiece of the Copenhagen Consensus analysis, and is used widely by the World Health Organization – yet it isn’t, and likely won’t be, the central metric in our analysis.
At this point I want to start a more thorough discussion of why this is. I’m going to start at the beginning, with a full description of what DALYs are (and the different ways of calculating them). Some readers will already be familiar with what’s below, but we want to make sure we clearly describe the metric and give examples of its implications before discussing its strengths and weaknesses.
The most complete account of DALYs I know of is in the Global Burden of Disease report. Page numbers below refer to this report.
The basics: burden of health problems in terms of years of life
A DALY is a measure of the “burden” of a health problem; two common uses of this measure are (a) ranking diseases and risk factors (from most to least burdensome), as the Global Burden of Disease report does, and (b) ranking different interventions (in terms of how much they can be expected to reduce burdens, “per dollar”), as projects including DCPP do. The basic DALY formula is on page 48:
DALY = YLL (Years of Life Lost) + YLD (Years of Life lost due to Disability)
YLL is the more straightforward component. Putting aside discounting/weighting issues (to be discussed later), the death of a male infant (life expectancy 80 years) would be counted as 80 years of life lost, while the death of a 45-year-old female (life expectancy 83.72 years) would be counted as 38.72 years of life lost (see page 402 for the life expectancy figures). Without further adjustments, this implies that the death of a single infant is considered about as bad in and of itself as the death of two adults.
YLD represents an attempt to convert years of life affected by a disability into the same terms as years of life lost due to premature death. For example:
- A year spent with blindness (as opposed to a year spent with “normal health”) is counted as 60% as “bad” (i.e., as much burden) as a year of life lost due to premature death. So the metric would count a condition that permanently blinds five 30-year-olds as about equally “burdensome” to a condition that results in the death of three 30-year-olds.
- A year spent with protein-energy malnutrition to the point of wasting (i.e., being severely underweight) is counted as 5.3% as “bad” as a year of life lost due to premature death. This implies that if a child is malnourished to the point of being severely underweight and having a lower life expectancy (say 30 years), the burden in DALYs is equal to about 51.59 (50 years of life lost due to early death; 30 years of malnutrition * 5.3% = 1.59 YLD), which is about 60% the burden of an infant death.
As for where these numbers come from (why is a year of blindness 60% as bad as a year lost, and a year of wasting 5.3% as bad?), they were obtained through a variety of methods usually involving surveying groups of people on their subjective attitudes (Pg 50 has more on this). The complete list of disability weights – giving a conversion factor for every kind of health condition analyzed by the GBD – is found on pages 119-125.
This basic framework – evaluating all health burdens in terms of “life-years,” with a year lost to death counted as a full year and a year otherwise afflicted counted according to the disability weights – is common to all DALY calculations. In the next post on this topic, I’ll discuss some of the variations between different versions of DALYs; some versions “discount” life-years that are early in a person’s life, late in a person’s life, or far in the future. After that, I will explain what we think the limitations of this metric are as it applies to our work.
- Copenhagen Consensus Center. Copenhagen Consensus 2008. http://www.copenhagenconsensus.com/Home.aspx (accessed April 15, 2010). Archived by WebCite® at http://www.webcitation.org/5p0sJczhJ.
- 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.
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