Lucky babies redux

Two babies born at 8:08 am on 8/8/08, weighing eight pounds, eight ounces, both in the United States.

How many would you expect?

The 2007 crude birth rate for the US is 14.2 per 1000, per year; the estimated US population is 304,843,316. The product of these is 4,328,775 births per year, or 8.25 births per minute.

From here I can find the distribution of birth weights (in Norway, 1992-1998 -- better figures would be appreciated). About six percent of babies weigh between 3850 and 3950 grams, which is a 3.5-ounce-wide interval; thus about 6%/3.5 = 1.7% of babies weigh 8 pounds, 8 ounces (to the nearest ounce) at birth.

So the expected number of babies born in the US at that particular minute, at that weight, is about 1.7% of 8.25, or 0.14.

There were two. The probability of this happening, assuming births are a Poisson process, is about one in 112. I wouldn't trust this number too much, because birth weights are supposedly growing with time and the Norwegian distribution is probably different from the US distribution.

So if I had to guess, people at the hospitals are fudging the numbers; if we were being totally honest, those babies might turn out to have been born at 8:09 am and weighed eight pounds, seven ounces, or something like that. Not that there's anything wrong with that.

(This post borrows a lot from a post I just remembered I made, lucky babies, about babies born on July 7, 2007 at 7:07 and weighing seven pounds, seven ounces -- but those were fictional babies.)