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Is consistency overrated?
  1. S Andrew Schroeder
  1. Correspondence to Dr S Andrew Schroeder, Department of Philosophy, Claremont McKenna College, Claremont, CA 91711, USA; aschroeder{at}

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In their insightful article, ‘The Disvalue of Death in the Global Burden of Disease’, Solberg et al argue that there is a potential incoherence in the way disability-adjusted life years (DALYs) are calculated. Morbidity is measured in years lived with disability (YLDs) in a way quite unlike the way mortality is measured in years of life lost (YLLs). This potentially renders them incommensurable, like apples and oranges, and makes their aggregate—DALYs—conceptually unsound. The authors say that it is ‘vital’ to address this problem, that ‘[n]eglecting [it] is not an option’, and that ‘one cannot add YLLs and YLDs together in [their] current form’.

Though one might object to their argument in various ways, let us assume the authors are correct that there is a potential inconsistency here. I want to ask why (or whether) we should be troubled by that. Now, this question may scarcely seem worth asking. Consistency and coherence are regarded as non-negotiable in so many domains that of course we should demand them in a measure such as the DALY. And this desire for coherence is clearly shared by both the architects of the DALY and their critics.2 3

I think, though, that things are more complicated. In 2012, I wrote an article discussing the epidemiological perspective from which DALYs should be calculated.4 Simplifying greatly, here is the problem: both YLDs and YLLs can be calculated either from prevalence data or from incidence data. The most natural way to calculate YLDs bases them on prevalences, because when we think about morbidity our initial thought is of people who are suffering. The most natural way to calculate YLLs bases them on an incidence measure, because when …

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  • i Could we say that, in attributing these dual goals, we thereby render the components commensurable under the common denominator ‘school excellence’? We could, but it would make commensurability very cheap, undermining the concern Solberg and colleagues identify. I will therefore focus on robust commensurability: commensurability along some dimension beyond the value we place in things.

  • ii In 2012, the Global Burden of Disease Study adopted the hybrid prevalence-incidence calculation method that I prefer, suggesting its authors agree.3

  • iii Murray alternately describes DALYs as true measures of health loss, or of the burden of disease.2 3 There are independent reasons to reject the former view, and the latter strikes me as vague enough to be interpretable in many ways, including as an index.

  • iv Murray (personal communication) has suggested that the DALY’s insistence on categorical causal attribution, according to which each adverse health outcome is (sometimes artificially) assigned to a single cause, may help to prevent advocates for particular issues from manipulating DALYs deceptively.

  • Contributors SAS is the sole author of this work

  • Competing interests None declared.

  • Provenance and peer review Commissioned; internally peer reviewed.

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