September 12th, 2012
(written by lawrence krubner, however indented passages are often quotes). You can contact lawrence at: firstname.lastname@example.org
But what do these numbers actually mean? You might guess from the first that someone born in the United States in 2009 could be expected to live about 78.5 years. This is not the case! It actually measures how long someone would be expected to live if every year of their life was spent in 2009. In other words, there is no accounting for progress that decreases mortality rates. And that’s on purpose. It is what is known as a “period life expectancy”.
Period life expectancies are used to track the general health of a population. With them you can easily compare one country to another. You can also monitor general population health over time. But the number you want if you’d like to know how long people will actually live is known as a “cohort life expectancy”. It measures how long someone born in a particular year (a cohort) can be expected to live. It is also not in the US Government yearly mortality report for 2009. The reason is that we won’t know it until everyone born in 2009 is dead! That will hopefully take a long long time. So, while we accurately know the period life expectancy for 2009, we do not yet know the cohort life expectancy for 1909. There are estimates of course, including one of about 83.9 years in 2009 from the Social Security Administration. I would like to offer, however, my own estimate of 86.4 years in 2009 and give the reasoning behind it. In particular the method of calculation reveals why life expectancy is likely still in the 80’s and not in the 120’s or 200’s like some would hope.
Life expectancy is calculated from something called a life table, which is just a big list of the probabilities of dying for someone of a certain age in a certain year. Below is plotted the one for 1940 and one for 2000. The period life expectancy for 1940 and 2000 would be calculated from exactly this data.