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# probability of same date of birth (Day and month)?

what is the probability of meeting some one randomly at a Mall or business place and finding the person has the same month and day of birth.

Should it be 1 /365 = 0.00273973 or we need to take into consideration the population present in the mall or business place?

Is there any database from any agency in USA or worldwide that reveals which is the mod (mostly occurring) day of birth in a year. Will http://en.wikipedia.org/wiki/United_States_Census have this information publicly available.

### 4 Answers

- 1 decade agoFavorite Answer
Technically, it is 1/365 at a mall or anywhere, but... take into account that more people will want to make love around Christmastime, New Years, Valentine's Day, a birthday, etc.

- michaelLv 41 decade ago
for your question, yes we do need to know the number of people at this place

consider this

the odds of someone having the same birth date in a room of 25

at a glace you would think 25/365, no??

but

thats the odds of one of the people in the room to have the same birth date as you

so take all possible pairings in the room

use the formula for combinations

25C2 = 25! / (2!(25 - 2)!) = 25 * 24 / 2 = 300 possible pairings

the odds for any one of those pairings to have the same birthdate is 1/365, so do 1/365 * 300 = 300/365 = 82% chance

summary, take a room of 25 people, there's an 82% chance of two having the same birthdate

- JoeLv 61 decade ago
if it's a random sample (as stated in your quesiton), then the probability is as you stated, 1/365

good luck