Disability adjusted life years is a measure the years of life lost (YLL) for each person who dies prematurely (before the age of 75 years) taking into account the quality of life. It is similar to quality adjusted life years (QALY) where the quality of the life is taken into consideration as well as the duration of life. In the calculation of YLL, if 1,000 people die from a particular medical condition, 10 of them prematurely at an average age of 67 years (eight years prior to age 75 years) then the average YLL per person would be eight years, and the total YLL over all persons would be 80. However, if disability adjusted life years is estimated instead, then a measure of disability if required. This is frequently estimated using statistical modelling. The DALY is the sum of the YLL plus the years of life lost due to a disability (YLD). This is the number of years lost through the person being disabled in some way by having the disease or medical condition.
Also see: Quality Adjusted Life Years, Years of Life Lost, Years of Life Lost Due to a Disability, and Life Expectancy (as healthy life expectancy and disability-adjusted life expectancy adjust life expectancy estimates for quality of life).