The Numbers Behind Life Expectancy, from Carl Bialik's "The Numbers Guy" column at the Wall Street Journal.
Michael Moore said in Sicko that life expectancy is higher in Cuba than in the U.S.; CNN says it's the other way around; it turns out that so much computation goes into these calculations that there's probably a substantial amount of error. You might naively think that if life expectancy is, say, 77 years, that means that the average person born 77 years ago (in 1930) is just now getting around to dying. But the problem is that medical care isn't static, so this doesn't tell us how long people being born now should expect to live. So what's actually done is that one looks at how many people of age N in, say, 2005 survive to age N+1 (in 2006), and then these are chained together to tell us how many people would live to, say, age 80 if medical care remained as it is today and so the mortality rates remained constant. Basically, life expectancy is a moving target, because medical care changes substantially during a single person's life.
However, although the number "77" might not be that meaningful, I would guess that differences between those numbers for different populations which had been computed in the same way are valid to look at. A society where this number is 80 is probably healthier than one where it's 70.
But as many people point out, the Cuban statistics might not reflect what's actually going on in that country. It's difficult to know for sure.
Also, this means that the low life expectancies for countries in sub-Saharan Africa which have been affected by the AIDS epidemic are probably lower than one would naively expect; one hopes that the AIDS epidemic won't keep killing people at the same rates that it is now.