你如果看懂预期寿命是如何算出来的，就知道答案了。我把你该看的给你拷出来，如果看不懂我也不再解释了。

 The most commonly used measure of life expectancy 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.^[1]