By U.S. Navy photo by Mass Communication Specialist 3rd Class Maddelin Angebrand [Public domain], via Wikimedia Commons

Breaking DALYs down into YLDs and YLLs for intervention comparison

Sebastian Farquhar and Owen Cotton-Barratt

Summary

Global public health remains a top contender for the best way to improve welfare through aid. Within health interventions, it is natural to allocate marginal spending to avert the most expected DALYs (disability adjusted life-years) per dollar.1

However, not all DALYs are the same and there are important differences between years of life lost (YLLs) and years lived with disability (YLDs). Not accounting for these categories separately may introduce a bias in decision-making because DALYs do not address non-health outcomes for individuals and the effects of outcomes on others. These effects are different in situations which involve primarily YLLs as opposed to YLDs. This analysis is of particular practical importance to effective altruists because two of the most promising interventions address different types of DALYS – deworming primarily averts YLDs and bed nets to prevent malaria primarily avert YLLs.2 This make us more inclined towards deworming as a top intervention than a naive evaluation would suggest. More sophisticated analyses incorporate terms that address many of these effects, but may still undervalue deworming relative to malaria nets.

This document is primarily written for people who already place a high weight on DALYs in identifying top public health opportunities. It does not argue for the use of DALYs, or consequentialist reasoning in identifying public health opportunities.

Overview of YLLs and YLDs in DALY calculations

The burden of disease, as calculated by the World Health Organisation in its 2010 Global Burden of Disease (GBD) assessment, divides DALYs into two categories – YLLs and YLDs. YLLs accrue when an individual dies. The gap in life-span between their age at death and the maximum life expectancy3, shown in dark blue in the graph below, contributes to the YLLs. The gap, while the person is alive, between full health and the degree of disability attributed to the condition, shown in light blue, contributes to YLDs. Note that the conditions which are assigned a ‘disability weighting’ and create YLDs include but are not limited to conditions that are traditionally thought of as ‘disabilities’.4

YLLs in dark blue and YLDs in light blue

Graph adapted from Gold M., Stevenson D., and Fryback D. 2002

YLDs and YLLs in malaria and schistosomiasis

Although malaria causes a great deal of morbidity, its largest contribution to the burden of disease comes from lives lost, primarily of children under five. Indeed, when evaluating the health impact of a charity distributing bednets, charity evaluator GiveWell explicitly only addresses under-5 mortality. The best estimate for YLLs caused by malaria is roughly 20 times greater than that for YLDs.

Parasites of the sort that deworming addresses, however, cause more harm by making people ill than by killing people. For schistosomiasis, a parasitic worm, the best estimate for YLDs caused is roughly 10 times greater than that for YLLs, although there is substantial uncertainty.

Malaria and schistosomiasis have different DALY profiles. Source: Global Burden of Disease 2010

DALYs are only about the individual

DALYs measure the impact of conditions on the health of individuals. However, if we are choosing between health interventions, and care about their benefits to broader society, we should also consider the effects of health interventions on others.

Many health conditions have significant externalities. More severe conditions can have large impacts on the lifestyles of families and friends. Transmissible conditions carry a very direct externality in the chance of spreading the condition to others. Death can be traumatic for those who knew the deceased. Even where one of these obvious mechanisms is not in place, a health condition which reduces the productivity of an individual will reduce the ability of a community to provide for its needs. Caring for individuals can also consume social resources that could have been spent elsewhere if those individuals were healthy. These effects are typically acknowledged as important, but are then ignored because they are hard to measure.

Without any good ways to measure these externalities, it might be reasonable to use the impact on the individual as a proxy for size of the total effect. However, it seems likely to us that the average size of the externality will depend on the type of direct health effect, and in particular may vary for YLLs and YLDs.

YLDs have larger associated externalities than YLLs

YLDs can have very large externalities. The cost of care and treatment can be very high even on a per person per year basis. In the developed world, life-time costs associated with managing conditions can be enormous. The life-time care costs related to the amputation of a leg in the US have been estimated at $509,275.5 The Global Burden of Disease 2010 assigns a weighting of 0.021 for amputation of a leg, long term, with treatment. So the health burden of an amputated leg would take more than 47 years to accumulate a single DALY, which is longer than the life expectancy of the average amputee. Since the marginal cost of DALYs, even in rich healthcare systems, is much less than this, the full impact of the condition will be substantially underestimated by only considering the health effects. In the developing world, although absolute costs of care are lower, the presence of conditions can dramatically reduce family incomes, both because individuals find it harder to work and because family members find their options for work restricted by the need to care for the individual.6 Because the costs of managing the conditions which create YLDs can be so high, the non-health benefits of avoiding that YLD can be high as well. This saving is not included in a cost-effectiveness analysis which only examines DALYs per dollar.

It is very difficult to weigh up the magnitude of the impact of the trauma and loss associated with YLLs. It seems likely, however, that these are often lower than the externalities we need to consider for health conditions which cause the same number of YLDs. An intervention that prevents the need for an amputation, for example, would avert as many DALYs as extending a life by less than a year. It is unlikely that the typical externalities of a death occurring a few months earlier than otherwise are on the order of $500,000 in scale, even in rich countries. Although there is clearly variation, as a rule of thumb YLDs are likely to be associated with larger externalities. Therefore, all else equal, we should prefer interventions that reduce YLDs. Because deworming primarily averts YLDs, this argues for preferring deworming to bed-nets when the costs per DALY are similar.

The externalities depend on age differently for each category

Health economists used to age-weight DALYs such that both YLDs and YLLs for the very old or very young were treated as less significant as those in the middle of life. However, in the current GBD framework there is no age-weighting. This is because DALYs represent only intrinsic health losses, and there is no principled reason why health should be more important at different ages. While this may be the correct approach to take for DALYs themselves, once we consider the indirect effects of health on other things we value, it becomes clear that age is relevant in choosing programmes. However, the effects are quite different for YLDs and YLLs.

YLDs in youth often have larger externalities

Where health conditions are present in young people the externalities are often larger. Young people are still developing rapidly and ill-health may negatively impact this development.7 These conditions can affect both individual well-being and also the process of accumulating human capital. Even where health conditions last for only a year in youth, they may have substantial effects throughout life which are not captured in health calculations. By contrast, a year with a condition which impairs development late in life will typically have a much smaller effect on the flourishing of the individual over their whole lifetime. Therefore, where interventions avert YLDs, they are better when they avert them younger, all else equal, and particularly in childhood. Deworming mainly averts YLDs in young people. The evidence for developmental effects of deworming in children could be improved by further investigation – but there is no disagreement that any such effects are stronger for children than adults.

YLLs in middle age often have larger externalities

The age-weighting used in previous versions of the Global Burden of Disease placed a high weight on DALYs created for young and middle-aged adults and a lower weighting on the very young and the very old. There are a variety of reasons for this sort of weighting, but many of these reduce to instrumental claims about the value of lost labour productivity, about the social disruption, or about the grief caused by a death.

Bereaved parents report stronger feelings of grief when losing older children than younger children, especially in resource poor contexts. Similarly, bereaved widows report feeling more grief when losing younger spouses.8 And adults of working age are more likely to have dependents, who may have a hard time adapting after their death. So the impact of death on others can depend on age, which should be a consideration in prioritising interventions.

From an economic perspective as well, the externalities of death in youth and middle age are higher than those at either tail. This is because society has expended some resources in developing human capital, but individuals still have a substantial period of continued productivity in front of them. It is not appropriate to consider this as a health effect of the death. However, when deciding which interventions to pursue it can be important to consider productivity losses, especially in comparatively poor contexts where economic growth has comparatively large welfare benefits. In these environments, parents may also depend on their children to look after them in old age, which means a child’s death later in life can be a pressing concern for their welfare.

As a result, where interventions mostly avert YLLs, one should prefer interventions that mostly avert deaths in young and middle-aged adults, all else equal. Malaria net distribution mainly averts YLLs due to infant and under-5 deaths. This makes us think it is a little less effective than the simple DALY calculation suggests.

Conclusion

An assessment of promising health interventions that cares about broader societal impact requires separate treatment of YLLs and YLDs. YLLs and YLDs cause different kinds of externality. This is relevant for a decision between two contenders for top interventions – deworming and malaria bed-nets – and may encourage us to prefer deworming. However, some of these effects are already modelled in intervention analyses which place less emphasis on DALYs averted.

1. For example, a simple reading of the work of the Disease Control Priorities Project 2nd edition, suggests such an approach. Some evaluators, such as GiveWell, place much less emphasis on averting DALYs.

2. You can examine the data at GBD Compare.

3. The maximum life expectancy used in GBD 2010 is based on the lowest mortality rate found world-wide for each age-bracket. It is therefore, at 86 years, higher than the current at birth life expectancy in most countries.

4. Indeed one of the complications in assigning the current DALY weightings is that, as people are instructed to consider only ‘health’ effects, conditions such as complete hearing loss, are assigned comparatively low weightings because they represent different ableness rather than ill health as such.

5. MacKenzie EJ, Jones AS, Bosse MJ, Castillo RC, Pollak AN, Webb LX, Swiontkowski MF, Kellam JF, Smith DG,Sanders RW, Jones AL, Starr AJ, McAndrew MP, Patterson BM, Burgess AR. Health-care costs associated with amputation or reconstruction of a limb-threatening injury. J Bone Joint Surg Am. 2007 Aug;89(8):1685-92. The costs reported in this study are slightly out of date and based on the USA. In general, treatment costs have not fallen over the period, although it is possible that shifts to lower cost interventions may have reduced the representative cost since publication.

6. For example see this survey by the World Bank.

7. Of particular relevance to deworming, for example, is the indication that increased deworming substantially increases future earnings and future health. Baird et al. 2011 found that between 2 and 3 additional years of deworming as a child increased earnings as a young adult by around a quarter, partly because individuals worked more hours and were sick less often and partly because they were able to move into higher earning paid labour.

8. Ball J. Widow’s Grief: The impact of age and mode of death. J. Death and Dying. 1976

Posted in CBA methodology, Prioritisation research, Research notes, Techniques.

4 Comments

  1. Hi Seb and Owen. Interesting and insightful article!

    I have a few queries. The first relates to my failure to understand one of your points. The second and third relate to a couple of factors I think are worth considering but you haven’t mentioned.

    Firstly, I’m not fully understanding the line of argument in the section ‘YLDs have larger associated externalities than YLLs’. I particularly get lost with the sentence “Since the marginal cost of DALYs, even in rich healthcare systems, is much less than this, the full impact of the condition will be substantially underestimated by only considering the health effects”

    If you could clarify this, it would be appreciated! However, I’m also unsure as to why you’re using the example of a leg amputation. Presumably it’s because this is an example where large externalities associated with YLD are generated. But surely the question is whether it’s true that deworming averts more YLD externalities than bednets?

    Secondly, I agree that ‘YLDs in youth often have larger externalities’ than in later life. And while ‘Deworming mainly averts YLDs in young people’, isn’t this also plausibly true of bednets? See, for example, the cognitive impairment on schoolchildren reported here:
    http://www.malariajournal.com/content/9/1/366

    I also wonder what the impact of discounting has on all this. If most of the benefits of deworming come from the individual benefits and externalities gained several decades into the future, this is less certain to occur than an intervention that has immediate life-saving impact.

    Thirdly, I agree that ‘YLLs in middle age often have larger externalities’ and this works against bednets if we’re considering bereavement and productivity. However, reducing child mortality seems to be a strong stimulant of fertility decline, which in turn seems to have positive economic consequences.

  2. Hi Matt,

    Thanks for the in-depth comments.

    On your first question, our intention is to understand the general relationship between YLLs and YLDs — is a DALY associated with either about equally bad, or is one systematically worse for society? We think this general question is important, as well as the specialised version which looks at deworming and bednets. The amputations provide an example that’s relatively tractable to think through.

    The sentence that confused you is saying that:
    1) The amount e.g. that the UK is willing to spend to avert a DALY is £30,000. There are unfunded opportunities to buy DALYs at a little over this threshold.
    2) The costs to society of treatment of a DALY coming from amputated limbs is around ten times this.
    3) Therefore the health effect is only a small fraction of the total impact of amputation. We should be willing to take opportunities to avoid amputations at a much steeper rate than £30,000/DALY, because we will get large benefits in avoiding future costs (even if these won’t be entirely borne by the health system).

    On your second point, it’s true that bednets have a significant effect of averting YLDs in young people, a large majority of the DALYs caused by malaria in the young are YLLs rather than YLDs (http://vizhub.healthdata.org/gbd-compare/), and this ratio is very likely similar for the DALYs averted by malaria. I absolutely agree with your comment about discounting; this will reduce the expected impact of averting YLLs, but probably not by all that much. I have a book chapter coming out on this topic; drop me an email if you’d like to see a draft.

    On the third point, I agree that fertility decline is a substantial benefit. I’m not sure that we understand all the mechanisms of benefit well enough there to quantify it precisely. My understanding is that the fertility decline approximately cancels out the child mortality, so that you end up with similar populations surviving into early adulthood before or after the intervention. You can informally think about how bad it is for the family’s welfare (in DALY-equivalents, ideally) to have an extra child and have that child die as an infant.

  3. “This analysis is of particular practical importance to effective altruists because two of the most promising interventions address different types of DALYS – deworming primarily averts YLDs and bed nets to prevent malaria primarily avert YLLs. This make us more inclined towards deworming as a top intervention than a naive evaluation would suggest. More sophisticated analyses incorporate terms that address many of these effects, but may still undervalue deworming relative to malaria nets.”I

    I found these 3 sentences very confusing, although I think I’ve got them figured out now. Because you hadn’t really started to argue that YLDs might be underestimated compared to YLLs, it was hard to judge which way this was supposed to read – whether deworming might be undervalued or overvalued.

    Are you saying:

    1) A naive evaluation would treate YLDs and YLLS equally, and lead us to undervalue deworming, because
    2) Deworming mostly prevents YLDs, and you think that these have greater externalities, so are more important to prevent than YLLs, and
    3) More sophisticated analyses try and take these externalities into account, but may not completely manage it, so still undervalue deworming a bit.

    Also, there’s a spelling mistake in the second sentence, and I don’t think your footnote links work.

  4. Thanks for a great article!

    I found the information in note 3 astonishing. If the DAILYs in use in comparisons, when dominated by YLLs, use this calculation, does not this point alone suggest that the interventions concerned are strongly overestimated?

    The following seems right.
    “From an economic perspective as well, the externalities of death in youth and middle age are higher than those at either tail. This is because society has expended some resources in developing human capital, but individuals still have a substantial period of continued productivity in front of them.”

    However, I was wondering, would people’s entering the workforce early, with little education, tend to reduce this effect? After all, I’d have thought that a signicant part of the ‘investment’ gone into the development of human capital occurs during pregnancy.

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