When there are 22 people in a room, what is the probability that two of them have the same birth days?

Printable View

- February 12th 2010, 05:40 PMPunchReal life probability...
When there are 22 people in a room, what is the probability that two of them have the same birth days?

- February 12th 2010, 07:42 PMRandom Variable
This is a classic problem referred to as the birthday problem

The probability that at least two people share a birthday is 1 minus the probability that no one shares a birthday.

Choose the first person. He can have any birthday. Now choose a second person. The probability that the second person doesn't' share the same birthday as the first person is 364/365 (excluding a birthday on Feb 29). Now choose a third person. The probability that the third person doesn't share the same birthday as the first or second person is 363/365. And so on.

P(none of the 22 people share a birthday) =

so P(at least two of the 22 people share a birthday) = - February 13th 2010, 01:04 AMmr fantastic
- February 13th 2010, 05:55 AMPunch
- February 13th 2010, 11:00 AMRandom Variable
Choose 2 people. That can be done in = 231 ways.

The probability that the two people share a birthday is 1/365. Chose a third person. The probabilty that the third person doesn't have the same birthday as the first two is 364/365. Choose a fourth person. The probability that the fourth person doesn't have the same birthday as the first two or the third is 363/365. And so on.

P(exactly 2 people share a birthday) =