What is the probability that two randomly chosen people will have been born on the same day?

Physicist: 0.0035% or about 1 in 28,500.

Two randomly chosen people.

The most recent, complete information I could find is the 2000 American census. There they have the number of people of a particular age sorted into five year blocks, so I assumed a “step function” for the age distribution (which isn’t terrible). Also, for the 85+ block, it turns out that 80 year olds live to 90 on average, so I figure the average number of days people live in the 85+ block is more or less the same as the others.

This qualifies as a “useless statistic”. It’s fairly wrong world wide, quite wrong when you consider the people you work with (probably), even more wrong at a high school graduation, and really wrong in a maternity ward. Even worse, it changes from year to year (well… decade to decade).

But it does line up well with the approximately 28,600 day average human life span (which is actually a little surprising).