What are the chances of sharing my birthday with a friend?
Being birthday buddies depends on the size of your friendship group – the more pals you have, the better the chances of having a joint birthday party.
There’s a 50 per cent chance that, in any group of 23 people, two people will share a birthday. First, we need to work out the probability that everyone in the group has a unique birthday.
Considering a ‘group’ of just one person, the chance is 100 per cent. If we add another person, there’s just one birthday that they could share with person one, so their probability of having a unique birthday is 364/365. For person three, it’s 363/365. And so on, until person 23, for whom the probability of a unique birthday is 343/365.
To find the probability of everyone in the group having a unique birthday, multiply all 23 probabilities together, giving 0.493.
So the probability of a shared birthday is 1 - 0.493 = 0.507, or 50.7 per cent.
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Robert is a science writer and visiting professor of science at Aston University.