The Birthday Problem

Why coincidences are common

An interactive lesson with 8 steps, about 7 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• Compute coincidences through the complement: the chance of no match.
• In 23 people, a shared birthday is more likely than not: there are 253 pairs.
• With enough opportunities, rare coincidences become likely.

Steps

1. A class of 30
2. Count the misses
3. Three people
4. The tipping point
5. Count the pairs
6. Pairs pile up
7. Truly large numbers
8. Your birthday