How Do You Work Out The Odds Of Two People In A Group Sharing The Same Birthday?

[sandyRoe]

This is prompted by the remarkable number of ABers sharing a birthday today.

[Buenchico]

I used to teach this stuff!
http://en.wikipedia.org/wiki/Birthday_problem#Calculating_the_probability

Probably the most important part of that page though is this graph, which illustrates the increasing probability of 2 people sharing a birthday as the group size gets larger:
http://upload.wikimedia.org/wikipedia/commons/thumb/e/e7/Birthday_Paradox.svg/300px-Birthday_Paradox.svg.png
It shows that you only need 23 people in a group for there to be a 50% chance of a shared birthday. Since there are usually just a few more than 23 people in each secondary school class, you'd expect roughly half the classes in a school to have a pair of pupils sharing the same birthday.

[sandyRoe]

The graph is interesting but the maths beyond me. I see in a group of just over 50 a shared birthday is nearly certain.

[sunny-dave]

That used to be a favourite 'ice breaker' when I ran training courses, Chris.

In my usual group-size of around 30, you could pretty well be certain that there'd be one (or more) shared birthdays.