Experts create life expectancy charts using a statistical tool called a life table. Researchers take current age- and sex-specific death rates and use the resulting values to determine life expectancy, according to the Australian Institute of Health and Welfare.
Death patterns in the population are what a person's life expectancy is based on. The lower the death rate in a particular population at a particular time, the higher the life expectancy. Life expectancy charts assume that the present death rate is going to persist throughout a person's entire life. Life tables are reliant on accurate data about deaths and the population, so they are reasonable estimates based on predictable data, says the Encyclopedia of Death and Dying. Life expectancy rates are only accurate if the person lives his life subject to the age-specific mortality rates of the entire population born in that year.
Some factors that contribute to a high life expectancy are low infant and child death rates, as well as an aging population that has access to quality health care. The life expectancy in most countries has been steadily increasing since the 1900s. This increase is due to the change in leading causes of death and illness, an increase in life expectancy for both newborns and the elderly and a decrease in low fertility, claims the National Institute on Aging. One of the most surprising trends has been the increase in the life expectancy of the oldest age groups. Between 1840 and 2007, there has been an average growth of three months of life per year.