实际上，预期寿命只是一个用来比较发展水平的指标。它的计算实际上有很大不确定性。
比如加拿大2015预期寿命82.2岁，指的是2015年出生的新生儿，按照当前的社会发展程度和综合死亡率，他们能活到82.2岁。
如果倒推82年，那么1933年出生的那批人，估计已经大部分死亡了。还活在世上的不会太多，不过如果还活着没死的话，也可能活的更长，毕竟医学和科学在不断进步。
For the US, we can see that life expectancy in 2005 was 77.6 years. This means that the cohort of infants born in the US in 2005 could expect to live 77.6 years, under the assumption that mortality patterns observed in 2005 remain constant throughout their lifetime. This is clearly a strong assumption—if you move the slider forward in the chart below, you’ll see that in 2010 the period life expectancy in the US was 78.8 years, which means that US mortality patterns did improve in the period 2005-2010.
比如加拿大2015预期寿命82.2岁，指的是2015年出生的新生儿，按照当前的社会发展程度和综合死亡率，他们能活到82.2岁。
如果倒推82年，那么1933年出生的那批人，估计已经大部分死亡了。还活在世上的不会太多，不过如果还活着没死的话，也可能活的更长，毕竟医学和科学在不断进步。
For the US, we can see that life expectancy in 2005 was 77.6 years. This means that the cohort of infants born in the US in 2005 could expect to live 77.6 years, under the assumption that mortality patterns observed in 2005 remain constant throughout their lifetime. This is clearly a strong assumption—if you move the slider forward in the chart below, you’ll see that in 2010 the period life expectancy in the US was 78.8 years, which means that US mortality patterns did improve in the period 2005-2010.
