I have to confess that I've never firmly understood life expectancy statistics. Razib has been putting up graphs on GNXP of life expectancy by county, such as this one of white male life expectancy (highest in southern Minnesota, lowest in the South, Oklahoma, and the Coal Belt). The maps are fun to look at, but when trying to make sense out of them, I realized that I don't understand the basics of what a life expectancy number even means. For example, where are people placed — where they were born, where they lived most of their lives, where they died?

Or, say, there are two rural counties that are very similar but one has an old folks home and the other doesn't, so many of the old people in the two-county region end up dying in just one of the counties. What impact does that have on life expectancy statistics by county?

And then a reader sent me a simple question:

I have a numeracy question for you because I don't know anyone else to ask. I have been thinking about life expectancy which I can easily determine for myself and for my wife. I know that she will probably outlive me, but how do I compute the probability that she will do so? (Yes, I know that it is strictly incorrect to assign a probability to a single event, but if we had 100 similar mes and 100 similar hers, then what?)

If you are less baffled by these matters than me, please comment below.