- Years of potential life lost
Years of potential life lost (YPLL) or potential years of life lost (PYLL), is an estimate of the
averageyears a person would have lived if he or she had not died prematurely.cite web|author=Gardner JW|coauthors=Sanborn JS|url=http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&list_uids=2083312&cmd=Retrieve&indexed=google|title=Years of potential life lost (YPLL)--what does it measure?|accessdate=2006-12-14] It is, therefore, a measure of premature mortality. As a method, it is an alternative to death rates that gives more weight to deaths that occur among younger people.
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  .
PYLL can be calculated using individual level data or using age grouped data. [cite web| last = Association of Public Health Epidemiologists in Ontario | title = Calculating Potential Years of Life Lost (PYLL) | url=http://www.apheo.ca/index.php?pid=190 | accessdate = 2008-05-13 ]
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 he or she dies, 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 - 60 = 15
# Reference age = 75; Age at death = 6 months; PYLL  = 75 - 0.5 = 74.5
# Reference age = 75; Age at death = 80; PYLL  = 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 emphasize the most common causes of death in older people, because the risk of death increases with age. Because PYLL gives more weight to deaths among younger individuals, it is the favoured metric among those who wish to draw attention to those causes of death that are more common in younger people.
For example, in most of the developed world, heart disease and cancer are the leading causes of death, as measured by the number (or rate) of deaths. For this reason, heart disease and cancer tend to get a lot of attention (and research funding). However, one might argue that everyone has to die of something eventually, and so public health efforts should be more explicitly directed at preventing "premature" death. When PYLL is used as an explicit measure of premature death, then the leading causes of death tend to be injuries and infectious diseases, rather than heart disease and cancer.
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