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Experiment to visualise the Birthday Paradox.

In probability theory, thebirthday problemorbirthday paradoxconcerns the probability that, in a set ofnrandomly chosen people, some pair of them will have the same birthday. By the pigeonhole principle, the probability reaches 100% when the number of people reaches 367 (since there are only 366 possible birthdays, including February 29). However, 99.9% probability is reached with just 70 people, and 50% probability with 23 people.