What is the birthday paradox?

Here’s a fun brain teaser: how large does a random group of people need to be so that there is a 50% chance that at least two of the people share a birthday? The answer is 23, which surprises many people. How is that possible?

Known in statistics as the “birthday problem” or “birthday paradox,” many people intuitively guess 183 because that’s half of all possible birthdays, since the year typically has 365 days. Unfortunately, intuition often fares poorly on this type of statistical problem.

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