At BBC Magazine, an examination of the “birthday paradox” using the World Cup as an example.

The birthday paradox goes like this: Mathematically, in any group of 23 people there is a 50% chance that two people will share a birthday. At the World Cup, there are 32 squads with 23 players on each team. Does the birthday paradox prove true there? It does:

“People think it’s an amazing coincidence that two people in a class of 30 share the same birthday,” he says. “Actually, with 30 people it’s a 70% chance.”

Or consider your favourite social media site. If you’ve got 70 friends, you’ve got a 99.9% chance that at least two of them will share a birthday.

But perhaps the best data-set of all to test this on is the football World Cup. There are 32 teams, and each team has a squad of 23 players. If the birthday paradox is true, 50% of the squads should have shared birthdays.

Using the birthdays from Fifa’s official squad lists as of Tuesday 10 June, it turns out there are indeed 16 teams with at least one shared birthday – 50% of the total. Five of those teams, in fact, have two pairs of birthdays.

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Photo: Wikimedia Commons