Variation matters (royal baby life tables)
In yesterday’s coronation lifetables post, it turned out that a 39-yr old UK male had about a 32% chance of outliving a 31-yr old UK male. That’s because there is quite a lot of variation around the mean life expectancy.
One might try estimating Ben Goldacre’s chance of seeing King George by working out Prince William’s life expectancy and then seeing how likely Ben is to be alive at that expected date. The calculation ignores the uncertainty in Prince William’s lifespan, and it ends up with a serious underestimate of 20%. If Ben Goldacre is still alive at baby George’s coronation, it is could well be because the coronation happens earlier than expected, and just using the mean life expectancy ignores this possibility.
The same issue arises in some of the tweets reacting to the StatsChat post, eg from James Shield “For me, about 70%, which seems high. My guess is I’ll be 86.” The 70% probability is so high precisely because the event could happen before he is 86.
Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »