Understanding DALYs

post by Paul Kelleher

Cost-effectiveness analysis (CEA) compares, you guessed it, costs with benefits. Costs are typically conveyed in monetary terms. Benefits are usually conveyed in terms of either QALYs or DALYs. QALYs are quality-adjusted life years. DALYs are disability-adjusted life years. QALYs have been around since the 1970s (as I understand it), whereas DALYs are a more recent innovation. In 1994, Christopher J. L. Murray, then of Harvard University, introduced DALYs in his paper "Quantifying the Burden of Disease." Murray then went on to the World Health Organization (WHO), where DALYs were used in the controversial attempt by WHO to rank the equity and effectiveness of the world's national health systems. Both QALYs and DALYs are used widely today as the units of benefit-measurement in cost-effectiveness analyses.

This week I've spent a lot of time reading and thinking about CEA, and I've encountered quite a bit of confusion in the literature on how DALYs should be understood. For example, I was looking at a paper entitled "The Economics of HIV/AIDS in Low-Income Countries: The Case for Prevention" which reports CEA results in terms of the cost per "loss of DALY averted." This implies that, whatever DALYs are, you should want more of them rather than fewer (i.e. you want to avert their loss). This understanding of DALYs is made explicit in "Using Cost-Effectiveness Analysis for Setting Health Priorities," a report by the Disease Control Priorities Project (which appears affiliated with the World Bank and WHO): 

In cost-effectiveness analysis, the DALY represents the number of years of disability-free life that would be gained from a particular health intervention—yielding a cost per DALY where cost data are available or can be inferred. Gaining a DALY through a health intervention reduces the burden of disease; it is the same as averting the loss of a DALY. 

Compare this understanding of DALYs with that stated in Disease Control Priorities in Developing Countries by Dean T. Jamison, who worked on WHO's Global Burden of Disease Project and continues this work at the at the University of Washington's Institute for Health Metrics and Evaluation (which Chris Murray now directs):

Disability-adjusted life years (DALYs) are a variant of QALYs in that they measure the losses from disability or premature death; a CEA will determine which interventions maximize QALYs or minimize DALYs.

So which understanding of DALYs is correct? Do we want to maximize DALYs (i.e. avert their loss) or do we want to minimize them (i.e. praise their loss)? The answer is that Jamison is right: DALYs are to be minimized (and their loss praised). But how could those others, who work with DALYs all the time, get it so wrong? Let me try to explain.

Both QALYs and DALYs are designed to integrate considerations of longevity and quality-of-life. For example, I generate 1 QALY for every year I live in full health. The number of QALYs one generates is calculated by multiplying the length of time (measured in years) by a "utility weight" which places one's health status on a scale of 0 to 1 where 0 is as bad as death and 1 is full health. So if I live 2 years in a health state that has a utility weighting of 0.8 (perhaps I have mild lower back pain), then I am said to generate 1.6 QALYs (2 x 0.8) over that time span. Given this understanding, one obviously wants more QALYs rather than fewer, since more QALYs correlates with more years lived in good health.

QALYs are therefore used to measure healthy years lived. DALYs, by contrast, are used to measure healthy years lost. That is, DALYs measure the gap between the number of healthy years one in fact lives and some normative ideal of how many healthy years equates to full health over a lifetime. The normative ideal of longevity is set using the world's longest actual life-span, which is set by the Japanese. The normative ideal of health-related quality-of-life is a trickier subject that I want to pass over here. The important thing is that it is stipulated by DALY-users that full health for each person is a certain number of healthy years of life. So when we suffer ill health and disability, that is regarded as stripping us of those healthy years. DALYs are used to measure the healthy years lost due to ill health.  

The confusion about DALYs enters when people treat DALYs as the healthy years that are lost to ill health. These folks then assume that successful health interventions work by clawing back those lost healthy years (or by "averting their loss" in the first place). Strictly speaking, however, DALYs don't get clawed back when health improves---they disappear or are obliterated. Thus, when one is in poor health, DALYs come into existence and displace one's stock of healthy life years (as set by the normative standard); but when one's health improves, one's newly regained healthy years of life displace or obliterate the DALYs that poor health created. Hence Jamison's dictate: minimize DALYs. One minimizes DALYs either by preventing the poor health that creates them in the first place, or by eliminating and substituting them with regained healthy years of life.

I hope that helps.

 

Comments

[Will]

Point of clarification: does this mean death @50 y/o increase DALYs by approximately 30 (as in 30 years short of life expectancy)?

Also, how do DALYs apply once someone has passed their life expectancy? Would someone dying at 90 have an increase in DALYs or not?

[Paul Kelleher]

Hi Will,

Using 80 years as the normative standard for life expectancy, death at 50 would generate 30 DALYs (other DALYs may have been generated while this person was alive; 30 is just the number created *by death* itself).--- DALYs use "disability weights" (contrast this with QALYs' "utility weights"). To get the number of DALYs in a life, you multiply the number of years lived in a given state of illness or disability by the disability weight assigned to that state of illness/disability. You do that for all periods of one's life (perhaps lived in different degrees of illness/disability), and all up all those numbers. Finally you add to that sum this number: [80 minus one's age at death]. This gives you total DALYs in a life.

Your second question is a good one. Remember, DALYs are years of healthy life lost. If I lived in full health up to 90 and then died, then I lived 90 healthy life years. But I can also live 90 healthy life years if I live to 100, as long as some of my years were lived in less than full health. The time spent in less than full health did not cut into my longevity, but it would've cut into my quality-of-life, and the metric of healthy life years takes this into account. So your question about someone who dies at 90 cannot be answered without knowing what this person's quality-of-life was. That said, if the normative standard for longevity is set at 80 years, and someone lives 85 years that, in light of their quality, generate 80 *healthy* life years, then I believe that no DALYs are generated by ill healthy this person might suffer after turning 85 years old.

I admit that there may be more to answering this second question than I personally know about.

[julian]

Life expectancies are defined at every age, so 80 is only the life expectancy at birth (e.g. in Japan). DALYs are normed against a full life table. At age 50, remaining life expectancy is (say) 34 years, not 30. At age 80, it is (say) 6 years. But it is always positive, so there are always DALYs on the table.

A quick note from my father (the Jamison in the post): "Phrasing the benefit as averting something definitely sows confusion. It may be useful to pass on that assessment of disease burden, ie the GBD, was a concept created at the World Bank and contracted by the Bank to WHO and Harvard to implement under the joint leadership of Alan Lopez and Chris Murray." He modestly leaves out that he (as noted in Paul's post) is the lead for the Disease Control Priorities Project.

[Paul Kelleher]

Thanks for the comment, Julian.

I actually didn't know (and didn't say in the post) that your father is the lead for DCPP. I hadn't investigated enough to know who was involved with DCPP. I just found that document via google and thought it a good contrast to the way your dad puts things in his book.

Thanks again.