Unusual.

Family have four children with same birthday at odds of more than 133,000 to one – Telegraph.

Parents Emily Scrugham and Peter Dunn were amazed when their newborn son arrived on January 12, the exact same date as his three older brother and sisters.

The couple from Cleator Moor, Cumbria, beat odds of a staggering 1/133225 in producing a quadruple birthday for their four children.

The fortuitous date was not planned as the twins were taken in an emergency operation and the other two did not have this as their due date. They were either early or late. Of course the twins were born on the same day so that makes only THREE birthdates. I’m thinking that reduces the odds a bit. Sure, it’s weird, but I suspect there is something about April as an amorous month for this couple which lends itself slightly towards January births. Anyway, I think it’s kind of neat (maybe not for the kids though).

My brother, his son and my younger son also have the same birthday. It is in early September, so they all have had years when the first day of school coincided with their birthdays. Weird, but it happens.

The odds are a bit off — given 4 random people, the odds that all 4 have the same birthday is (approximately) 1 out of 48 million (the odds given are the odds that 3 people have the same birthday, not 4). In any case, that’s also a bit misleading, because those are the odds of 3 specific people. The odds that there exists a family with 3 (or in this example, 4) children have the same birthday are significantly better. The larger your data set, the greater the odds.

Yes. I thought the odds looked a bit on the low side but wasn’t sure how to work it out.

Just a note on due dates. As I understood (being father of 1 plus 2×1, the idea of a due date is a bit misleading. The gestation period is not a fixed number of days, but an approximation with a reasonable margin of error being plus or minus two weeks from conception. Would this not make the odds hard to work out?

Four words they’ll never hear their parents say in all seriousness “When’s your birthday, again?”

Even if the odds of this happening were 48 million to one, then given the number of families there are in the world, the chances that you would find such a family are close to a certainty. Not so?

It wouldn’t be a certainty, but it’s not surprising either. I’m not as good at math as I used to be — its been decades since I’ve had to know this stuff.

I (vaguely) remember a math seminar I attended in college. The speaker had done research on gambling, and his main observance was that in reality streaks (ie. coincidences) happen FAR more often than people expect them to. Pretty much, the whole gambling industry depends on our false expectations. The same concept, though, applies to other forms of coincidences, such as people sharing the same birthday. The reason why they happen is that the world is big and you have a wide selection of data to find those coincidences in. To the family where it occurs, of course it’s amazing and completely surprising. But that such a family exists is less so.

My girlfriend and her sister have the same birthday, March 15th at one year apart. And my girlfriend’s two daughters also have the same birthday, Dec 10th about five years apart.