History of Burden of Disease Studies
In 1992, the World Bank commissioned the initial GBD study to provide a comprehensive assessment of the disease burden in 1990. The study was undertaken for the world as a whole and for 8 regions ( Lopez and Murray 1998 ; Murray and Lopez 1996a , d ; Murray, Lopez, and Jamison 1994 ; World Bank 1993 ). In order to recommend intervention packages for countries at different stages of development, the estimates were combined with analyses of the cost-effectiveness of interventions in different populations ( World Bank 1993 ; Jamison and Jardel 1994 ). Whereas earlier attempts to quantify global cause of death patterns ( Hakulinen and others 1986 ; Lopez 1993 ) were valuable initial contributions to building the evidence base for policy, they were largely restricted to broad cause of death groups, for example, all infections and parasitic diseases combined, and did not address nonfatal health outcomes.
The methods and findings of the 1990 GBD study have been widely published and, as noted earlier, have spawned multiple disease burden exercises ( Murray and Lopez 1996c , d ; 1997a , b , c ).One of the basic principles guiding a burden of disease assessment is that almost all sources of health data are likely to contain useful information provided they are carefully screened for validity and completeness. With appropriate methods, investigator commitment, and expert judgment, obtaining internally consistent estimates of the global descriptive epidemiology of major conditions is possible. To prepare internally consistent estimates of incidence, prevalence, duration, and mortality for almost 500 sequelae of the diseases and injuries under consideration, a mathematical model, DisMod, was developed for the 1990 GBD study to convert partial, often nonspecific, data on disease and injury occurrence into a consistent description of the basic epidemiological parameters in each region by age group ( Barendregt and others 2003 ; Murray and Lopez 1996b ).
Many diseases, for example, neuropsychiatric conditions and hearing loss, and injuries may cause considerable ill health but no or few direct deaths. Therefore separate measures of survival and of health status among survivors, while useful inputs when formulating health policy, need to be combined in some fashion to provide a single, holistic measure of overall population health. To assess the burden of disease, the 1990 GBD study used a time-based metric that measures both premature mortality (years of life lost because of premature mortality or YLL) and disability (years of healthy life lost as a result of disability or YLD, weighted by the severity of the disability). The sum of the two components, namely, DALYs, provides a measure of the future stream of healthy life (years expected to be lived in full health) lost as a result of the incidence of specific diseases and injuries in 1990 ( box 1.1 ). The effect of fatal cases (of disease or injury) is captured by years of life lost, while YLD captures the future health consequences in terms of sequelae of diseases or injuries of incident cases in 1990 that were not fatal. (For a more complete account of the DALY measure and the philosophy underlying parameter choices, see Murray 1996 ; Murray, Salomon, and others 2002 ).
[Box 1.1]
DALYs are not unique to the GBD study. The World Bank used a variant of DALYs in its seminal review of health sector priorities ( Jamison and others 1993 ), and they are derived from earlier work to develop time-based measures that better reflect the public health impact of death or illness at young ages ( Dempsey 1947 ; Ghana Health Assessment Project Team 1981 ).
Much of the comment on, and criticism of, the GBD study focused on the construction of DALYs ( Anand and Hanson 1998 ; Hyder, Rotllant, and Morrow 1998 ; Williams 1999 ), particularly the social choices pertaining to age weights and severity scores for disabilities. Relatively little criticism was directed at the vast uncertainty of the basic descriptive epidemiology for some populations, especially in Sub-Saharan Africa (see chapter 5 in this volume), which is likely to be far more consequential for setting health priorities ( Cooper and others 1998 ).
The results of the 1990 GBD study confirmed what many health workers had suspected for sometime, namely, that non-communicable diseases and injuries were a significant cause of health burden in all regions, and in some rapidly industrializing regions such as East Asia and Pacific, were already by far the leading cause of death and disability. Neuropsychiatric disorders and injuries in particular were major causes of lost years of healthy life as measured by DALYs, and were vastly under-appreciated when measured by mortality alone. The original GBD study estimated that noncommunicable diseases, including neuropsychiatric disorders, caused 41 percent of the global burden of disease in 1990, only slightly less than communicable, maternal, perinatal, and nutritional conditions combined (44 percent), and that 15 percent of the burden was due to injuries. Earlier assessments of global health priorities based on mortality data attributed no deaths to mental health disorders and less than half (7 percent) of that suggested by DALYs to injuries ( Lopez 1993 ).
Estimates of the disease and injury burden caused by exposure to major risk factors are likely to be a much more useful guide to policies and priorities for prevention than a "league table" of the disease and injury burden. In recent decades, researchers have attempted to quantify the effects of specific exposures, for instance, tobacco smoking, on mortality from major diseases such as cancers ( Doll and Peto 1981 ; Parkin and others 1994 ) or from multiple diseases ( Peto and others 1992 ; United States Department of Health and Human Services 1992 ), either in individual countries or across groups of countries using comparable methods.
Specific country studies have examined the impact of several leading risk factors (Holman and others 1988; McGinnis and Foege 1993 ), but prior to the 1990 GBD study, no global assessments of the fatal and nonfatal burden of disease and injury resulting from exposure to multiple major health risks had been attempted. The 1990 study quantified 10 risk factors based on information about causation, prevalence, exposure, and disease and injury outcomes available at the time. The study attributed almost 16 percent of the entire global burden of disease and injury to malnutrition; another 7 percent to poor water and sanitation; and 2 to 3 percent to such risks as unsafe sex, tobacco, alcohol, and occupational exposures ( Lopez and Murray 1998 ; Murray and Lopez 1996a ; Murray and Lopez 1997a ; Murray, Lopez, and Jamison 1994 ; World Bank 1993 ).
