Is anyone good at statistics / probability and want to help?

[Darkestseasonofall]

I'm born on the same date as my MIL, and we both gave birth to daughters on the same date. I find this a bit uncanny, but can't work out what the odds are, just idly musing really.

According to this you need a group of 23 people to have a 50/50 chance of the same birthday and 75 to have a 99.9% chance.

I would have thought that you and MIL and your DD and MIL DD would be around 1 in 75 x 1 in 75, maybe even less uncommon so about 1 in 5600 or about 1 in 20000 that you both had daughters too, so not massively common, but not lottery winning odds.

Could well be a situation that affects a few thousand families in the UK, same birthday for two people in extended family and two other people in extended family with same birthday.

Not quite the same, but a colleague has the same birthday as my brother and DPs cousin has the same birthday as my sister.

Taking it as a totally random, the chances of you sharing the same birthday as your MIL would be 1/365, so it is quite unusual. However, with say 12 million married women in the UK, you'd expect around 33,000 of them to find that they shared a birthday with their MIL.

The chances of your daughter having the same birthday as your MIL's daughter is again 1/365. (Assuming she only had one daughter - if she had two daughters, you've got two chances for a match so it would be 2/365).

As a thought experiment, if say 5,000 of those wives I mentioned in the first paragraph had a MIL who had birthed a daughter and then had a daughter themselves, you'd expect around 14 of those wives to produce matching daughter birthdays.

So, you are very unusual, but I suspect there will be a few other women in this country who have the same statistical claim to fame!

I should have said, if a woman married at random a man who had one sister, then had a daughter, the probability that she would share her birthday with her MIL and the daughter shared her birthday with the SIL would be 1 / (365 x 365) = 1/133225.

I've just re-read your OP. Above, I interpreted it as saying "you & MIL - share a birthday" then "MIL's daughter & your daughter - share a birthday", but that those are different birthdays.

If instead you are saying that you, your MIL, her daughter and your daughter all share the same single birthday, then that is really unusual, around 1 in 50 million - I reckon that would be a few women worldwide.

Sorry, my OP is confusing.
So MIL and I are born on March 1st, our daughters are born on June 6th, so the 4 of us don't share a birthday.

That's fine - as I say, that's how I interpreted it, so my first two posts on this thread remain valid and there could easily be a dozen other women in this country who can make the same claim, a few thousand worldwide. I wonder if any of them will see this thread?

DadDadDad has covered it.