Uncertainties about disability weights for the Global Burden of Disease study
Authors: Erik Nord
Journal: The Lancet Global Health (2015)

In the Global Burden of Disease (GBD) 2010 study,1Murray CJ Ezzati M Flaxman AD et al.GBD 2010: design, definitions and metrics.Lancet. 2012; 380: 2063-2066Summary Full Text Full Text PDF PubMed Scopus (847) Google Scholar disability weights for a large number of diseases and disabilities were estimated from judgements of health states elicited in household surveys in Bangladesh, Indonesia, Tanzania, Peru, and the USA, and in an open access web-based survey.2Salomon JA Vos T Hogan DR et al.Common values in assessing health outcomes from disease and injury: disability weights measurement study for the Global Burden of Disease Study 2010.Lancet. 2012; 380: 2129-2143Summary Full Text Full Text PDF PubMed Scopus (984) Google Scholar In a useful update by Salomon and colleagues,3Salomon JA Haagsma JA Davis A et al.Disability weights for the Global Burden of Disease 2013 study.Lancet Glob Health. 2015; 3: e712-e723Summary Full Text Full Text PDF PubMed Scopus (725) Google Scholar participants in the GBD project show that judgments in four European countries are mostly quite similar to those obtained in the previous study. Salomon and colleagues also show that judgments are quite robust as to whether disorders are described as chronic or temporary. For 30 of the health states in the 2010 study, descriptions were revised in the 2013 study. As a result, valuations of some of the states changed substantially. In particular, deafness, in 2010, was assigned the implausible weight of 0·03, and being in a wheelchair with a spinal cord lesion below the neck 0·05. I suggested that this was due to the GBD being about losses in health and respondents thinking that people with the two disorders had disability rather than poor health.4Nord E Disability weights in the Global Burden of Disease 2010: unclear meaning and overstatement of international agreement.Health Policy. 2013; 111: 99-104Summary Full Text Full Text PDF PubMed Scopus (63) Google Scholar The GBD responded by adding information in the 2013 study about health effects of the two disorders. The weights then became 0·22 for deafness and 0·30 for being in a wheelchair with a spinal cord lesion below the neck, which is still debateable, but much more plausible than before. From these results, Salomon and colleagues conclude that "in some cases, responses are evidently highly sensitive to particular details in these descriptions". Such sensitivity is both natural and desirable. A large number of diagnoses in the 2010 study were presented in subcategories representing different degrees of severity and treatment status. Differences in wording and indications of effects produced huge differences in responses across subcategories, and rightly so. But, because valuations are sensitive to severity, two points are of importance in calculations of the population burden of a disease. First, one should know how people with the disease are distributed between main subcategories of severity. Second, disability weights in the GBD study should pertain to descriptions that fit with these main subcategories. Absence of good data for severity distributions, and absence of fit between severity categories in disease statistics and severity categories for which disability weights are given in the GBD, could substantially reduce reliability of population disease burden estimates. Salomon and colleagues note that "vast and ambitious new data collection" is needed to meet this challenge. In the GBD surveys, respondents are presented with descriptions of pairs of people in different states of illness and asked "who of the two is healthier". The phrase is the result of an aim in the GBD project to measure health losses in terms of their size.1Murray CJ Ezzati M Flaxman AD et al.GBD 2010: design, definitions and metrics.Lancet. 2012; 380: 2063-2066Summary Full Text Full Text PDF PubMed Scopus (847) Google Scholar, 2Salomon JA Vos T Hogan DR et al.Common values in assessing health outcomes from disease and injury: disability weights measurement study for the Global Burden of Disease Study 2010.Lancet. 2012; 380: 2129-2143Summary Full Text Full Text PDF PubMed Scopus (984) Google Scholar, 3Salomon JA Haagsma JA Davis A et al.Disability weights for the Global Burden of Disease 2013 study.Lancet Glob Health. 2015; 3: e712-e723Summary Full Text Full Text PDF PubMed Scopus (725) Google Scholar Several writers4Nord E Disability weights in the Global Burden of Disease 2010: unclear meaning and overstatement of international agreement.Health Policy. 2013; 111: 99-104Summary Full Text Full Text PDF PubMed Scopus (63) Google Scholar, 5Nord E Menzel P Richardson J Multi-method approach to valuing health states: problems with meaning.Health Econ. 2006; 15: 215-218Crossref PubMed Scopus (5) Google Scholar, 6Hausman DM Health, well-being, and measuring the burden of disease.Popul Health Metr. 2012; 10: 13Crossref PubMed Scopus (19) Google Scholar have argued that the focus on size is meaningless. When people rank people with different kinds of health problems, they cannot avoid applying subjective value judgments of the importance of different dimensions of health. Disability weights should therefore be understood as valuations of health losses—ie, judgments of their undesirability—rather than quantifications. Quantification is simply without empirically verifiable meaning. Countries wishing to use burden of disease statistics in health planning might be interested in doing surveys to test whether ranking of disorders in the pairwise comparisons in the GBD 2013 study fit with those that their own population would make. Absence of clear meaning from the phrase "who is healthier?" could compromise survey respondent's willingness to respond or the reliability or validity of their responses. In Norway, pilot studies at the Norwegian Institute of Public Health have used phrasings in terms of undesirability rather than "who is healthier?" to make questions meaningful. Salomon and colleagues have not attempted to address the fundamental conceptual critique of their question framing. Disability weights are derived from ordinal measurements of preferences (paired comparisons of health states), but are themselves supposed to have ratio scale properties. Transformation of ordinal data into weights is achieved by means of advanced modelling, the validity of which was difficult to assess from very dense statistical information provided in the 2010 study.4Nord E Disability weights in the Global Burden of Disease 2010: unclear meaning and overstatement of international agreement.Health Policy. 2013; 111: 99-104Summary Full Text Full Text PDF PubMed Scopus (63) Google Scholar The 2013 study represents little improvement on this account. Ultimately, the only convincing way to validate the disability weights is to apply the so-called proof of the pudding lies in the eating test. For instance, if a disability weight for state A is 0·4 and for state B is 0·2, the implication is that a disease causing 2000 people to live 1 year in state B is as undesirable from a public health point of view as a disease causing 1000 people to live in state A for 1 year. Another implication is that a disease causing 100 deaths, each associated with a loss of 10 life-years, is as undesirable as a disease causing 5000 people to live in state B for 1 year (100 × 1 × 10=5000 × 0·2 × 1). The question is whether people and policy makers in countries where the weights are supposed to be used agree with these implications. I miss simple validation studies of this understandable kind in the GBD project. I declare no competing interests. Disability weights for the Global Burden of Disease 2013 studyThis study substantially expands the empirical basis for assessment of non-fatal outcomes in the GBD study. Findings from this study substantiate the notion that disability weights are sensitive to particular details in descriptions of health states, but robust to duration of outcomes. Full-Text PDF Open Access