Submitted by tensai on

Saw a post at badscience.net which piqued my interest. Before you read it, first think about the odds of a mother having 3 children on the same day of the year. How likely (or unlikely, as the case may be) would you guess it is? Turns out it's a lot more likely than most people would think. OK, now you can click the link.

FWIW, thanks to my recently revived statistical knowledge, I did pick up that the correct odds wouldn't be 365*365*365, but rather just 365*365. That's because once the first kid is born, the chance of him being born on that day is exactly 1 (unless she's a Chinese gymnast in which case she'd be born 3 years later). Think of that the next time someone asks, "what are the chances of that happening again?". The answer is, the same as the chances that it happened in the first place (assuming the factors haven't changed, of course).

I also realized that the odds of same day births would probably be lower due to the dates of conception being non-independent. For instance, my wife and I planned to have our kids in the spring because she didn't want to be pregnant during the heat of the summer. So the range of possible days our kids could have been born is only a window of about 100 days. Would you believe they were all born inside that window? (Actually, one of them did sneak in a bit late, but within a decent margin of error).

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