Concept
Years of potential life lost (YPLL) or potential years of life lost (PYLL), is an estimate of the average years a person would have lived if they had not died prematurely. It is, therefore, a measure of premature mortality. As an alternative to death rates, it is a method that gives more weight to deaths that occur among younger people. An alternative is to consider the effects of both disability and premature death using disability adjusted life years. To calculate the years of potential life lost, the analyst has to set an upper reference age. The reference age should correspond roughly to the life expectancy of the population under study. In the developed world, this is commonly set at age 75, but it is essentially arbitrary. Thus, PYLL should be written with respect to the reference age used in the calculation: e.g., PYLL[75]. PYLL can be calculated using individual level data or using age grouped data. Briefly, for the individual method, each person's PYLL is calculated by subtracting the person's age at death from the reference age. If a person is older than the reference age when they die, that person's PYLL is set to zero (i.e., there are no "negative" PYLLs). In effect, only those who die before the reference age are included in the calculation. Some examples: Reference age = 75; Age at death = 60; PYLL[75] = 75 − 60 = 15 Reference age = 75; Age at death = 6 months; PYLL[75] = 75 − 0.5 = 74.5 Reference age = 75; Age at death = 80; PYLL[75] = 0 (age at death greater than reference age) To calculate the PYLL for a particular population in a particular year, the analyst sums the individual PYLLs for all individuals in that population who died in that year. This can be done for all-cause mortality or for cause-specific mortality. In the developed world, mortality counts and rates tend to emphasise the most common causes of death in older people because the risk of death increases with age. Because YPLL gives more weight to deaths among younger people, it is the favoured metric among those who wish to draw attention to those causes of death that are more common in younger people.