1. Measuring the Global Burden of Disease and Risk Factors, 1990—2001

The DALY is a health gap measure that extends the concept of potential years of life lost due to premature death to include equivalent years of healthy life lost by virtue of individuals being in states of poor health or disability ( Murray 1996 ). One DALY can be thought of as one lost year of healthy life and the burden of disease as a measure of the gap between current health status and an ideal situation where everyone lives into old age free from disease and disability. This conceptualization of DALYs as a measure of health, and not of lost utility, is analogous to the principles of measuring gross domestic product as summarized by Eisner (1989 , p. 7): "Our focus is on measures of all economic activity related to welfare [for example, gross domestic product], but not of welfare itself." Information on calculating DALYs, on time discounting, and on age weights is provided in chapter 3 .

DALYs for a disease or health condition are calculated as the sum of YLL in the population and YLD for incident cases of the health condition. YLL is calculated from the number of deaths at each age multiplied by a global standard life expectancy for the age at which death occurs. To estimate YLD for a particular cause for a particular time period, the number of incident cases in that period is multiplied by the average duration of the disease and a weight factor that reflects the severity of the disease on a scale from 0 (perfect health) to 1 (dead). The weights used in the 2001 GBD study are listed in detail elsewhere (see annex tables 3A.6 to 3A.8 in chapter 3 ).

In addition, in calculating DALYs, the GBD study used 3 percent time discounting and non-uniform age weights which give less weight to years lived at young and older ages. For the results reported in this volume and used in the Disease Control Priorities in Developing Countries , second edition (DCP2) 3 percent time discounting was applied but not non-uniform age weights. A death in infancy then corresponds to 30 DALYs, and deaths at age 20 to around 28 DALYs. Thus a disease burden of 3,000 DALYs in a population would be the equivalent of around 100 infant deaths or to approximately 5,000 persons aged 50 years living one year with blindness (disability weight 0.6).