ArticlesCommon values in assessing health outcomes from disease and injury: disability weights measurement study for the Global Burden of Disease Study 2010
Introduction
The Global Burden of Diseases, Injuries, and Risk Factors Study (GBD) aims to quantify health losses from a wide array of diseases and injuries. These losses are expressed in units of disability-adjusted life-years (DALYs), which account for both premature mortality—measured as years of life lost (YLLs)—and time spent in states of reduced health—measured as years lived with disability (YLDs). Although the term disability has many meanings in different contexts,1, 2, 3, 4 in the GBD, disability refers to any short-term or long-term loss of health.5
For the latest revision of the GBD (GBD 2010), YLDs have been computed for 1160 sequelae resulting from 289 disease and injury causes, by multiplying the number of people living with each sequela by an associated disability weight. Extensive efforts were made to standardise and streamline the list of health consequences across diseases, and as a result the 1160 sequelae have been mapped into a set of 220 distinct health states that capture the most salient differences in symptoms and functioning.6 The disability weight for a health state is a number on a scale from zero to one that represents the severity of health loss associated with the state. A value of 0 implies that a health state is equivalent to full health, and a value of 1 implies that a state is equivalent to death.
The previous comprehensive estimation of the global burden of disease (undertaken in the final revision of GBD 1990, which was published in 19967) used the judgments of a small group of health-care professionals to establish disability weights for 483 sequelae of 131 diseases and injuries. These disability weights were used widely in WHO's revisions of the GBD for 1999–2002, and 2004,8, 9, 10, 11, 12, 13, 14 and in several national and subnational burden of disease studies.15, 16, 17, 18, 19, 20, 21, 22 Additions and amendments to the 1996 GBD weights have been assimilated selectively, largely on the basis of the Dutch Disability Weights study,23, 24, 25 which adapted the GBD measurement approach from the 1996 study, with specific modifications to the descriptions of health states and addition of several states.
In view of the widespread use of GBD weights and the centrality of disability weights to the comparable measurement of disease burden across diverse causes, there has been much commentary and debate about the 1996 GBD weights and their derivatives, with critics challenging several aspects of the work.26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40 Four broad topics dominate the debate: how to define the construct being measured; what methods of measurement of this construct to use to elicit responses from individuals or groups; whose responses should be elicited; and how universal the resulting weights are.
The starting point for measurement of disability weights should be a clear definition of the construct to be measured. Two distinct possibilities are to measure welfare loss or health loss. Welfare loss is a broad construct, and while health outcomes undoubtedly affect wellbeing generally, there are probably many additional influences from factors unrelated to health. A debate in health economics continues about whether to take overall wellbeing or health as the quantity to be maximised in health-policy choices,41, 42, 43 with a parallel discussion in philosophy addressing the so-called separate spheres argument about appropriate accounting for health and non-health consequences in evaluation of health-care priorities.44 However, a broad recognition that health might be afforded a special status that distinguishes it from other elements of wellbeing pervades many strains of discourse in public policy and international law. For example, protection of the right to health was most famously articulated in the International Covenant on Economic, Social and Cultural Rights, which affirmed “the right of everyone to the enjoyment of the highest attainable standard of physical and mental health”.45 Most governments have ratified international treaties that include the right to health,46 and provisions for health or the right to health appear in 135 national constitutions.47 Although some have argued that the burden of disease should be quantified in terms of overall welfare loss because health and wellbeing are not separable,48 others have challenged this view.43, 49, 50
The measurement of disability weights in GBD 1990 did not clearly distinguish between health and wellbeing,7 but an evolution in the conceptual thinking behind the GBD has subsequently made explicit the aspiration to quantify health loss rather than welfare loss.51 This choice now distinguishes the GBD from other strains of research into weighing of health consequences; therefore, we draw attention to important implications of this difference when relevant.
A related set of issues concerns the specific measurement methods used to elicit judgments about health or welfare from individuals or groups. Measurement of disability weights in the 1996 GBD revision7 was partly based on a technique called the person trade-off, which locates comparisons between health outcomes within a resource allocation framework. Broad debate about methods pertains not only to measurement of the burden of disease, but also to related efforts to quantify outcomes in economic evaluations of health interventions.52, 53, 54, 55 Key issues range from technical concerns about the psychometric properties of different measures55, 56 to conceptual and ethical concerns about some methods, including the specific formulation of the person trade-off in the 1996 GBD weights.26, 57 Methods appropriate for eliciting judgments about welfare loss should be adapted to study health loss.42 There are ongoing debates about the relevance of specific values and judgments, such as time preference, risk aversion, or inequality aversion that could complicate the interpretation of responses to different elicitation methods.52, 58, 59
A third question in discussions of disability weights is whose judgments should be used to derive these weights. Three respondent groups have been considered: health-care professionals, individuals who experience a health state, or the general public. Arguments for various respondent groups are both principled and pragmatic.60, 61, 62, 63 The 1996 GBD weights7 used health-care professionals on the basis that they would have knowledge of a diverse set of health states and would be able to make comparative judgments. Individuals in a health state have the most intimate knowledge of the reductions in function associated with that state; however, their comparative judgments with other health states will be based on asymmetric information.62, 64 Additionally, the capacity of individuals with some chronic disorders to adapt to their circumstances could lead to underestimation of the health loss associated with a particular state.7, 65, 66 Much of the scientific literature about health-state weights uses the responses of the general public, on the basis of the argument that, in a democratic society, the views of the general public are relevant in comparative assessments that inform public policy.61, 67
Finally, the universality of disability weights has been much discussed. A central theme in some critiques of DALYs and in broader discussions of disability has been the contextualisation of disability within a particular social and cultural environment,3, 36 which raises questions about the possibility of significant cross-cultural variability in disability weights.27, 28, 30, 68 The universality of disability weights could largely depend on the specific construct that is chosen. A reasonable hypothesis is that the construct of health loss associated with different health states is more universal than is the construct of welfare loss. Welfare loss might be strongly affected by social context, support networks, and a myriad of individual preferences that might be less pertinent to measures of health loss. For example, the same health loss could be associated with different welfare losses in societies with disability and health insurance compared with those without such insurance. Fundamentally, questions about the universality of measures of health loss and welfare loss are empirical ones, but the evidence needed for systematic scientific investigation of these questions remains limited.
Beyond the disability weights measurement study from the 1996 revision of GBD 1990, several other investigations of disability weights have added to both the empirical basis for weighting of health outcomes and the conceptual and philosophical debates about these measures. Some of these studies have focused on estimation of weights for many conditions with an adaptation of the 1996 GBD approach, as in the Dutch study,23, 24, 25 the European Disability Weights Project,68, 69, 70 and some national burden of disease studies.71, 72, 73 Several others have focused on techniques to obtain these weights in culturally diverse settings.74, 75, 76 Various studies have provided critical perspectives and new empirical measurements for disability weights pertaining to specific disorders or categories—eg, depression,77 suicidality,78 stroke,79 injuries,80, 81 oral health,82, 83 and neglected tropical diseases.39, 84, 85, 86, 87
For GBD 2010, we have undertaken a comprehensive re-estimation of disability weights through a large-scale empirical study. This report describes the design, implementation, and results of the GBD 2010 disability weights measurement study, which yields novel disability weights with measurements of uncertainty for the 220 unique health states arising from the array of disease and injury causes in the GBD. Addressing the four themes of persistent debate and responding to critiques of previous efforts to measure disability weights for the GBD, we focused on eliciting judgments about health loss rather than welfare loss; used a new, standardised approach to measurement with simple paired comparison questions; included a major emphasis on surveying respondents from the general public; and used primary data collection in diverse communities to examine hypotheses about cultural variation in assessments of disability.
Section snippets
Study design and participants
The study was done through a multicountry household survey and an open-access web-based survey. Household surveys were done between Oct 28, 2009, and June 23, 2010, in five countries (Bangladesh, Indonesia, Peru, Tanzania, and the USA) that were selected to provide diversity in language, culture, and socioeconomic status. All household surveys were administered as face-to-face, computer-assisted personal interviews, except for the survey in the USA, which was administered with computer-assisted
Results
Table 1 shows the characteristics of the 13 902 participants in the household surveys and the 16 328 participants in the web survey. The web survey included respondents from 167 countries, 27 of which had at least 50 respondents (figure 1). 7180 (44%) participants in the web survey were from the USA. 10 579 (93%) of 11 320 selected respondents in the four face-to-face household surveys participated, and the probability of response was at least 88% in each site. 3323 (69%) of 4833 selected
Discussion
This study, to our knowledge, is the largest empirical effort to date to measure disability weights for a wide array of health outcomes across a diverse range of populations. Several important findings emerged. First, we have demonstrated that it is feasible to elicit assessments about a wide variety of health outcomes in virtually any population, even where educational attainment is low. Second, we have established the utility of a new approach to elicit this information with much simpler
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