Previously, I outlined the basics of the Disability-Adjusted Life Year (DALY) metric. It takes the approach of converting all health burdens into equivalent “years of healthy life lost”: a year of blindness is counted as .6 lost years, a year of severe malnutrition is counted as .053 lost years, etc.
This post discusses two common “variations” on DALYs, meant to deal with relatively thorny disagreements about how different years of life should be valued. As before, page numbers refer to the Global Burden of Disease 2000 report.
One variation has to do with the intuition some people have that a 20-year-old’s death is more tragic than an infant’s. (I expressed this intuition myself back in November, and I still hold this view.) In an attempt to square with this intuition (which is common and well-documented, as Pg 400 shows), the DALY metric includes an optional age weighting feature that lowers the value of a healthy year of life lived at very young and very old ages, relative to the value of a healthy year of life around age 20. DALYs can be computed with or without age-weighting (“without” just means that all years of healthy life are valued the same).
The other variation has to do with valuing present vs. future benefits of aid. DALY calculations apply a discount rate to future benefits; for example, when using a discount rate of 3%, one would count a year of healthy life saved ten years from now as being worth only 74% as much as a year of healthy life saved this year (74% = 1/1.03^10).
I confess that I don’t fully follow the justification for discounting given in the Global Burden of Disease Report, which claims that “the strongest argument for discounting is … [that] not discounting future health would lead to the conclusion that all of society’s health resources should be invested in research programs or programs for disease eradication” (400), which apparently is considered obviously wrong by the authors. Personally, the most appealing argument I can think of for discounting is that helping a person can help them help others, so helping a person sooner is literally “worth more” than helping a person later.
DALYs(0,0) refers to DALYs calculated with a 0% discount rate and no age-weighting. DALYs(3,1) refers to DALYs calculated with a 3% discount rate and age-weighting. (The first number in parentheses is the discount rate; the second is a 1 if age-weighting is being used, and a 0 if not.) See Pg 401 for the specifics of how varying these numbers affects the valuation of different years.
In theory, you can calculate DALYs using whatever parameters best fit your own philosophical values. In practice, the reports we’ve seen using this metric (Global Burden of Disease Report, Copenhagen Consensus, Disease Control Priorities Project) will give you, at most, DALYs(0,0), DALYs(3,0) and DALYs(3,1), and will rarely give you the inputs into these numbers so you can calculate your own versions. That means that if you want to use a 6% discount rate, you’re completely out of luck; there’s no way to convert DALYs(3,0) to DALYs(6,0) without having more information. More importantly, it means that:
- You can’t use your own version of age-weighting. Even the age-weighted version of DALYs still rates an infant death as about equally tragic to a 20-year-old death (it values a year more for a 20-year-old, but when you work it all out the value of a life comes out the same). There is evidence (see pg 401) that people find a 20-year-old’s death to be far worse; if you share that intuition, then DALYs as they are usually presented won’t reflect your values, and there will be no way to convert them into a unit that does.
- You can’t use your own disability weights. Personally, this is the area I’d most like to see some variation in – the official disability weights disagree violently with my personal intuitions about, for example, how bad it is to be severely malnourished (current weights put it at only 5.3% as bad as a year of life lost – see Pg 121) or how bad it is to go through an abortion (it appears that this is counted as “no cost” by DALYs – see Pg 121 again).
The DALY metric does have some flexibility to accommodate different personal values, but in practice it ends up being pretty rigid. More on this in a future post.
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