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Not long after joining Significance, having taken up residence in the Royal Statistical Society's (RSS) headquarters, I discovered that I shared my birthday with another member of staff. 'What are the chances?', I was tempted to ask. But, of course, I knew better. While doing my background reading for the new job, I stumbled across the 'birthday problem' several times.

Many Significance readers will be familiar with the birthday problem. But for those who aren't, it concerns the probability of two people, within a randomly chosen group, sharing the same birthday.

It seems unlikely when we experience it, especially among a small group of people. But among the staff of the RSS, of which there are 25, the likelihood of two people sharing a birthday is a little over 50%. Take a group of 100 people, and the probability rises to almost 100%.

I was reminded of the birthday problem as statisticians (and others) prepare for Huntrodds' Day tomorrow – a celebration of chance, coincidence and randomness.

Here at Significance, to mark the occasion, we're making two articles from our archive freely available to all. Both concern the birthday problem. They are:

And, by pure coincidence, our upcoming October issue features an in-depth interview with author and statistician David Hand about his new book, The Improbability Principle, which explains why extremely improbable events are actually fairly commonplace.

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