 Life expectancy is a statistical measure of the average time an organism is expected to live, based on the year of its birth, its current age and other demographic factors including gender. The most commonly used measure of life expectancy is at birth alibi which can be defined into ways. Cohort alibi is the mean length of life of an actual birth cohort all individuals born at given year and can be computed only for cohorts born many decades ago, so that all their members have died. Period alibi is the mean length of life of a hypothetical cohort assumed to be exposed, from birth through death, to the mortality rates observed at a given year. National alibi figures reported by statistical national agencies and international organizations are indeed estimates of period alibi. In the Bronze Age and the Iron Age, alibi was 26 years, the 2010 World alibi was 67.2 years. For recent years, in Swaziland alibi is about 49, and in Japan, it is about 83. The combination of high infant mortality and deaths in young adulthood from oxidants, epidemics, plagues, wars, and childbirth, particularly before modern medicine was widely available, significantly lowers alibi. But for those who survive early hazards, a life expectancy of 70 would not be uncommon. For example, a society with an alibi of 40 may have few people dying at precisely 40, most will die before 30 or after 55. In populations with high infant mortality rates, alibi is highly sensitive to the rate of death in the first few years of life. Because of this sensitivity to infant mortality, alibi can be subjected to gross misinterpretation, leading one to believe that a population with a low alibi will necessarily have a small proportion of older people. To for example, in a hypothetical stationary population in which half the population dies before the age of 5 but everybody else dies at exactly 70 years old, alibi will be about 36, but about 25% of the population will be between the ages of 50 and 70. Another measure, such as life expectancy at age 5-5 can be used to exclude the effect of infant mortality to provide a simple measure of overall mortality rates other than in early childhood, in the hypothetical population above, life expectancy at 5 would be another 65. Aggregate population measures, such as the proportion of the population in various age groups, should also be used along individual-based measures like formal life expectancy when analyzing population structure and dynamics. Mathematically, life expectancy is the mean number of years of life remaining at a given age, assuming age-specific mortality rates remain at their most recently measured levels. Three of his denoted by backslash display style he underscored XA which means the mean number of subsequent years of life for someone now ages backslash display style X, according to a particular mortality experience. Longevity, maximum lifespan, and life expectancy are not synonyms. Life expectancy is defined statistically as the mean number of years remaining for an individual or a group of people at a given age. Longevity refers to the characteristics of the relatively long lifespan of some members of a population. Maximum lifespan is the age at death for the longest-lived individual of a species. Moreover, because life expectancy is an average, a particular person may die many years before or many years after the expected survival. The term maximum lifespan has a quite different meaning and is more related to longevity.