Birthday Probabilities

[Drewcifer]

From the "How old are you?" thread in the OT forum:

Me and Ewo, too. This is getting a little scary.

Yeah, it is a little weird. I did some research online though, and for a group of people this size (the ~90 that voted), there is a 99% chance that two people will have the same birthday. I'm not sure what the chances are for two groups of people to have the same respective birthdays, but probably close. Unfortunately, all of the birthday probability studies I found disregarded the birthyear, so I have no idea what the chances of having the exact same birthday are. Much less, I'd guess.

Hmmm....
0.99*0.99 is not very high, but we could narrow it down by finding out exactly how many pairs of poeple share the same birthday.

As for years, our age ranges span roughly 20 years, but are mostly within a 10 year bracket.
Lets say there are 100 of us, two pairs of which share the exact same birthdate.
There are 20*365 days that this date can be = 7300
If my birthday is 11/02/81 (dd/mm/yy, because it's more logical), than the chances of Ewo, picked at random, of having the same birthday is 99/7300 = 0.0136
Of the 98 left, your birthday is the same as Jegs2, so the probability of that is 97/7300, =0.0133
0.0136*0.0133 = 0.000181
which is 0.02%

The same calculations given a 10 year age range gives 0.07% - nearly four times that

But the ages are not evenly distributed acorss the range, they give normal distribution.

Someone want to correct my maths? It's been too long...

Tell you want - Christmas puzzle I'll set my parents (Mum's doing a maths degree). We'll work out what the actual probablilty is.

Who else shares exact birthdates?

I thought it was interesting (and worthy of a thread in SLAM) that of the ~100 people that have voted (as of Dec 16) in age thread, at least two pairs of us have the exact same respective birthdates.

[Alferd Packer]

While you'd have to examine lots of population data for various countries, it may also be that in certain countries, people tend to be born more often in certain months and certain days than others.

[TrailerParkJawa]

There is also a rise in birthrates after significant events. I saw on TV that 9 months after the great North Eastern Black Out of 1965 there was an increase in births.

As for the board, I was born on the same day as Weemando but not the same year.

[XPViking]

Okay. My math is a little rusty but this is how I see it:

I currently live in a city of 350 000. How many are born in the same month?

350 000/12 = 29 167

How about the same day?

29167/30 = 972

How about same year?

972/80 (average human lifespan) = 12

Please correct me if I'm way off here. In my life, I've only met one person who shares the same birth year, month, and day. We also happen to be good friends. I think the other question is: what are the odds of you meeting someone like that?

XPViking

[SyntaxVorlon]

THAT depends entirely upon the personalities of the two people, the locations(or probability that they will live or be in eachothers proximity), and so many other variable that Chaos theory begins to be applicable.