The United States ranks number 42 among the world's 224 recognized nations, with an average life expectancy of 79.56 years, according to The World Fact Book. In the United States, men's life expectancy is 77.11 years, and women's life expectancy is 81.94 years.
Life expectancy is measured as an estimate for individuals born in a given year, presuming that the current death rates for each age remain constant for the duration of said individuals' lives. These death rates are then averaged. Therefore, a life expectancy figure is the mean of death rates for all ages in a particular demographic. Demographers officially refer to this metric as "Life Expectancy at Birth."
Monaco holds the number one ranking position, with a total life expectancy of 89.57. Monacan men have a life expectancy of 85.66 years, and Monacan women have a life expectancy of 93.64 years. The country of Chad ranks lowest, at number 224. Total life expectancy in Chad is 49.44 years, with men's life expectancy at 48.3 years, and women's life expectancy at 50.63 years.
The life expectancy of a country is a critical indicator of its general condition, and it is used by demographers to further assess development. The Human Development Index is a metric, developed by the United Nations, by which the overall well being of a given country's population can be determined. Many criteria are factored to produce a country's HDI ranking, including life expectancy. While HDI ranking and life expectancy ranking are based on two entirely different measurements, there is a correlation between the two.