The construction of the daly: implications and anomolies

The disability-adjusted life year (DALY) is a measure of aggregate ill-health whose construction depends on a counterfactual – the number of life-years a person could have expected to live had she or he not died. There are two ways of specifying the DALY counterfactual to estimate years of life lost (YLL) – by employing an ‘exogenous’ or an ‘endogenous’ life table. An exogenous life table is independent of the mortality risks experienced by the population whose health (longevity) is being assessed, whereas an endogenous life table is composed of precisely these risks. Exogenous life tables have been used to construct the DALY in the Global Burden of Disease (GBD) studies – with different exogenous life tables used in the GBD 1990 and GBD 2010 (and later) exercises. However, an endogenous life table is more appropriate for predicting life-years lost from premature mortality in any given country, and allocating resources through health interventions there on the basis of DALYs averted. Whether an exogenous or an endogenous life table is used, anomalies can arise. Furthermore, the approach adopted in GBD 2010 onwards adds special difficulties of its own. GBD 2010 and later GBDs use an exogenous reference life table which is the same for men and women. This leads to an underestimation of the disease burden of women relative to that of men