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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}cmc.edu

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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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