Life expectancy is a statistical measure of how long an organism may live, based on the year of their birth, their current age and other demographic factors including sex. At a given age, life expectancy is the average number of years that is likely to be lived by a group of individuals (of age x) exposed to the same mortality conditions until they die. The most commonly used measure of life expectancy is life expectancy at age zero; that is, at birth (LEB), which can be defined in two ways: while cohort LEB is the mean length of life of an actual birth cohort (all individuals born a given year) and can be computed only for cohorts that were born many decades ago, so that all their members died, period LEB is the mean length of life of a hypothetical cohort assumed to be exposed since birth until death of all their members to the mortality rates observed at a given year.