A Statistician's Birthday

So have you ever checked out what happened in history on your birthday thinking somehow the one day out of 365 that you were randomly born is more important than the other 364?  And if there are 364 people in a room with you what is the probability that someone has YOUR date for a birthday (after all it is YOUR date not theirs)?

Try almost 100%.  In fact the probability is 50% with only 23 people in a room and 99% with 57 people (remember it is month and day only--- not the year).

 

Errata:  10/2/12 -  I'm embarrassed to say I made the same mistake as Johnny Carson (see Blog Erroneous Company).  You actually need 1679 people in the room to have a 99% chance that at least one person matches your exact day of birth (month and day).  The 57 people in a room for 99% probability answers a different question - "How many people do you need in a room for anyone's birthday to match (meaning two or more 'coincidentally' have the same birth day)".

 

And how many people were born on the same date as you?  The most common birth date in the USA is October 5^th – back up 9 months and you get Jan. 1.  And the least common birth date – May 22.   But the most common birth months are August, July and September.  You come up with the hypothesis. 

 

And to be really precise – your birthday occurs once every 7 years (just like those born on Feb. 29).  Not counting leap years, the date you were born moves each year to the following day of the week.  Your annual celebration is actually for your birth date – not your birthday.

 

In Indian Hill Schools, they celebrate the half-birthdays for kids like Jenna and Ellen who don’t get to have a birthday celebration during the school year.  Soon we will be able to send half-birthday cards to each other.

 

 

And the most famous and celebrated birthday of all – December 25 (depending on the calendar you use).