The Birthday Problem...
Does anyone have the same birthday as you? Of course they do. I share mine with Olivia Newton-John, Bryan Ferry and Will Self. But how about within your group of friends?
When I was in the first year of secondary school our maths teacher asked us a question: in a group of randomly chosen people, how big will the group need to be before there is more than a 50/50 chance that some pair of them will have the same birthday?
We all guessed. Wildy. Some said 365. Some said 183. I was second closest though with a guess of 50.
The actual number is surprisingly low. Just 23. That is to say if you have just 23 people in group there is a 50/50 chance that two of them with share the same birthday.
Now this birthday problem asks whether any of the 23 people have a matching birthday with any of the others — not one in particular. To find out the size of the group in which there is a 50/50 chance of someone having the same birthday as you the maths is a bit different and the answer is much bigger. 253.
So it depends of the question you ask. I don't know any people in my group of friends who have the same birthday as me but I do know people within my group of friends who share the same birthday. Which stacks up with the odds.