Life expectancy is defined as the average lifespan between a given age and death. Mathematical formulas exist that can determine individual life expectancies and the life expectancy of a larger population. In the latter case, one can calculate life expectancy from a population’s life table.
This chart is compiled through analysis of statistics. The mortality rate, or death rate, is determined for each group in a population. Subsequently, the mortality rates are used to compose a graph known as a survival curve. From this graph, one can calculate life expectancy. The life expectancy formula is taken by first calculating the probability of a population’s survival at every year in the life table.
Let’s say that a life table reflects a population of 1 million people beginning in 1940. If, by 1941, 100,000 of those people have died, 900,000 remain. Therefore, the death rate for Year 1 is 10 percent and the probability of survival is 90 percent, or 0.90. When one adds all of these probabilities for each subsequent year — up to and including the maximum age on the life table — then one can calculate life expectancy results. Statisticians often add a half year to the overall tally, because most people live at least halfway past their final birthday.
Various factors can influence the life expectancy calculation. An ideal life table for a population would have a large representative sample and cover at least 100 years to allow for aberrant fluctuations and comprehensive results. Yet most life tables represent a much shorter time period, leading to more variable estimation measures for individual years and, thus, greater room for error. Other statistics that can skew a population’s life expectancy are infant mortality and elderly mortality. Death rates among infants prior to the age of 1 and adults in old age — above 70 years — are higher than any other age sets across populations, so these numbers can adversely impact a population’s overall life expectancy if too much emphasis is placed on them.
Calculating life expectancy has several uses. Since the era of ancient Rome, human beings have sought to determine their life expectancy in some form. In modern times, the formula was developed to aid insurance companies in setting their rates. Contemporary applications range from determining the overall quality of life and health of a population to determining lifestyle differences in various subsets of a population.
Scientists have utilized the ability to calculate life expectancy in deriving a number of important findings. For example, life expectancy tends on average to be higher among females, those of a higher socioeconomic bracket, and Caucasians and Asians. Medical and industrial advances in many parts of the world during the past century also have significantly heightened populations' life expectancy in numerous countries. As of 2010, some estimates put world life expectancy at age 67.2, an increase of 20 to 30 years since the beginning of the 20th century.
Buster29 Post 2 
My uncle worked for the life insurance industry for over 30 years, and he told me there were people who spent their entire workday entering raw information on all the people who had died that week. The insurance company's experts could just look at a customer, get a little personal information, and calculate his or her life expectancy. Most of the time, they were right within a few years.
My dad was just about to retire from his job, so my uncle used a retirement calculator like he would with any other client. My dad wasn't just his brother at that time. He was a 65 year old Caucasian nonsmoker with a family history of diabetes and hypertension. All of these things factored into his life expectancy after retirement. 


Cageybird Post 1 
I can see where the average life expectancy in a developed country like the United States would be considerably longer than the average life expectancy in a Third World country. Most people in the United States have access to decent medical care, clean water and nutritious food. All those factors and more probably go into life expectancy tables.
I'd like to find a chart showing life expectancy by country to see if there are any surprise findings. I'll bet there are some developed countries where life expectancies are lower than I expect, and some smaller countries where people live longer than I thought. 