The terms “life expectancy” and “lifespan” describe two distinctly different things, although people tend to use these terms interchangeably.

Life expectancy refers to the number of years a person is expected to live, based on the statistical average. This statistical average is calculated based on a population overall, including those who die shortly during childbirth, shortly after childbirth, during adolescence or adulthood, those who die in war and those who live well into old age.

A number of factors influence life expectancy including gender, race, exposure to pollution, education status, race, income level and healthcare access. Modifiable lifestyle factors such as exercise, alcohol status, smoking status and diet also influence life expectancy. Therefore, life expectancy is highly variable from one individual to another. However, epidemiologist and statisticians still note trends and patterns in terms of life expectancy across data sets obtained for various geographical areas.

Lifespan, on the other hand, refers to the maximum number of years that a person can potentially expect to live based on the greatest number of years anyone from the same data set has lived. Taking humans as the example, the oldest documented age reached by any living individual is 122 years, meaning humans are said to have a lifespan of 122 years.

In mathematical terms, life expectancy refers to the expected number of years remaining for an individual at any given age. In formulaic terms, life expectancy is denoted by ex, where “e” represents the expected number of years remaining and “x” represents the person’s present age.

One argument that exists holds that life expectancies should be predicted after a person’s childhood, when it is possible to get a better handle on lifespan. For example, the “Roman Lifestyle Expectancy” table, demonstrates how life expectancy is significantly different after childhood. At birth life expectancy was 21, whereas by time a child reached 5 years of age, life expectancy would extend to 42 years. Other studies such as “Dead at Forty,” and “Plymouth Plantation,” have also demonstrated the significant jump in life expectancy that is predicted once adulthood is reached compared with childhood ages.